PC turns on but nothing happens

Line breaks will certainly help you.

And also showing your logs from event viewer.

After 10 years there is a good chance something has broken atleast.

Have you tried reformatting? If you don't have any important files you might want to try it. Most of the time it fixes the problem.
If bios isn't showing up, it might be a ram problem. If problem still occurs after reformating try installing OS in another storage if you have one. Might just be a harddrive problem.

check teh ram sticks and disconnect the usb+audio cables from the mother bored , leave only the important stuff ( sata cable + MB and cpu power cables ) also check the gpu

Can you go to the bios (by pressing DEL key)? If yes, likely a hard disk problem or OS. If you can acquire a Linux boot able drive, you can salvage your files then proceed to reformat or replace hard disk

If even bios is not accessible, try removing ram. Should have beep code error.
Try removing GPU also if that doesn't work
Also try resetting the cmos

It's hard to tell, but replacing stuff with spare ones laying around would make it easier. It could be the hard drive as it's an old pc I assume, the processor or a faulty component on the motherboard. Sometimes things work well when they are not stressed or on lower clocks, but stop otherwise because they are close to breaking.

How full is your drive(s)? It is quite possible you may have exceeded capacity, and could likely experience the drive failing. It is best to remember to keep your drives half full because you require as much space as your RAM - up to twice as much - for storing your pagefile. In light of that, you may want to look for crash dump files as they can take up considerable amounts of space, and delete them if they are present. (I had an issue where they became so huge, the dump files started overwriting my partitions that had separate Windows version installs and Linux installs. It was a mess!) You can locate these memory.dmp files either in C:\Windows\ or in C:.

Another thing you can do to keep a mechanical HDD healthy is to reformat every 6 months. Not that I ever do. 😜

I saw that many have suggested formatting the HDD and saying its faulty, but HDD is obviously not at fault here.
The thing is you get "black screen", which can be GPU, CPU, RAM or Motherboard issue.
Easiest diagnosis would be to put in spare parts, but as you have stated you dont have any.

So, if your motherboard has internal GPU, then try pulling out your external GPU and using the one on motherboard, if it is not at fault then try RAM, if you have multiple sticks of RAM, then try testing them one by one, if RAM isnt an issue, then its either CPU or Motherboard, but the thing is those two can only be tested by spare parts.

If only you had internal speaker, it could have been easily diagnosed by the beeping sounds it would produce when you turn on the PC.

Do a Bios reset per mainboard jumper/button. Check the mainboard manual how to.

from your information.
its either
HDD;Mainboard,Power Supply,Windows.Ram.USB,Wires inside the pc at fault .
Check your HDD with tools atleast ,or exchange the HDD with another one.
But first you should if you get in again
Remove all programms from Autostart (programm in autostart pays a heavy toll on starting the pc and can cause Black screens and loading times of 10min-1h in alot of cases):
Or you could if you have use a save point back in tiime ,when the Laptop was running normally .
remove your ramsticks boot the pc will it start ,put the second ram stick first and test .than put your first one slot 1 on second test . if you have 2
It could be that the Power supply doesnt get enough Electricity or it doesn t supply enough for the mainboard .(some old Pc have no safety feature that works.
removing all USB parts from your keyboard while starting and see if the Pc launches w/o problems ,USB Plug and play can have sometimes some impact on the start (Or if one of them is defect).
btw after such long time its possible that one pc part is loose or that the one cable isnt sitting perfectly in place you could check that out too .

last resort would be installing windows new .
Does your Pc beepto teel you the problem on starting?if yes how many times and how long is the beep?

FYI: Usually GPU Errors have different Issues than Black screens like Shaky screen ,Red screen or pixelating on the screen or lines appearing on the screen atleast in my experience .

Because i don't seen it mentioned before...

Maybe only the Monitor have a problem or the cable to the monitor isn't anymore fully correct connected.
Deconnect the cable on the PC and Monitor and connect it then again.
And if you have the possibility, then exchange the cable with a other one.

A good friend (a IT tech. :-D ) had exactly this cable problem on a LAN, and he had a HDD that he expected to die "soon" -so that was his first guess-, and he changed all stuff on the PC for hours to find the error, nothing helped on his place. So they decided to bring the PC to a other room, with prof. equipment, to work on it, checked the voltage of the components etc., all were fine and he had a picture on a other monitor. So he were happy and thought they found the error and it were maybe only some part that were not fully connected but is now. He gone back to his place, connect all and again black screen. First that brought them to the idea with the monitor and/or cable and in the end the cable was, somehow, the problem

Besides that, it sounds as you have no second monitor, no second PC etc. so your "possible tries to find the error" are very limited right now with a black screen, so i advice that you go to a repair store.
They have the parts and equipment to check, fast, what make the problem. And then you can decide if you do/let them do something or not.

I would remove anything plugged into your PC except the monitor, keyboard and mouse. If it's running on WiFi I'd shut that down as well. I'd unplug the monitor from the outlet as well as the PC and then reconnect it. If your pc ends up booting up I'd run a Checkdisk.

If don't know how to do this run a command prompt as administrator and then type CHKDSK C: and hit enter. Of course if C isn't your main drive put the one that is your main drive.

I know the code has long since been used but what is the answer for "!!"?

Initial feeling would be that your Hard Drive was starting to fail. The weird part thou is the symptoms you are having in the very end sounds like a power supply issue.

When a Power supply (or part of it) fails, you can press the power and hear fans spin and some lights on the case, but if the power rails to the CPU is the part that failed, you will not be able to boot.

The worst part is it can be a number of things. It gets even harder to troubleshoot if there are no bios error codes or beeps. You could range from almost any part of your computer having a failure and to track it down may requiring testing each part in another machine, or working parts from another marching in your computer to eliminate possibilities.