My pc doesn't start up.

That's a hard one as I misplaced my crystal ball. First off I'd check if all connectors are properly seated and the PSU switch usually on the backside of the PC is in "on" position. If that doesn't work out you'd best proceed with testing your setup by swapping out your parts with other known working components step by step. Edit: I forgot to mention that "cleaning" isn't as harmless as it seems as all to often a careless move can knock off a tiny surface mounted device (SMD) from the mainboard or GPU rendering them a paperweight.

Depends on what he did, did he disconnect any connectors inside the case, from motherboard, from GPU, PSU, if he did I would check all of those and reconnect properly.

Sometimes RAM needs to reconfigure itself after there is no power, which can take up to few minutes until it finishes and start booting up properly again, I would leave PC on for 10 minutes, which would be more than enough for it to finish, if this is indeed what is happening.

If your PC has that tiny speaker inside that beeps, it can tell you what is wrong, by how many beeps it makes, or LED lights on motherboard if your motherboard has it (I don't really know since you haven't shared your configuration)

Could be the motherboard having a dead component somewhere.

Its always the clean up.
2 years ago i short circuited my mobo because of a loose screw.
Ended up going to my friends house to test my gpu and cpu.
Had to buy a new asus motherboard that was also was dead on arrival.

He didn't wash it in the dishwasher, right?

Your brother knows what he did best, it's really a gamble at this point that could be anything.
If there is another pc, swap components and figure it out one by one.\

You hear no beeps starting up?

Ram ?

The motherboard should have indicator lights (some use the power button light) or an LED display. Check you motherboard manual. Then systematically remove everything from the motherboard (SSD, GPU, RAM, CPU) and replace them one by one checking the motherboard codes to see if it indicates where the fault is.

Does your processor has onboard video (like Intel Graphics with an "F" in the end of the version number)? You could try to connect with it, since you said the GPU doesn't turn the fan on. Or if you have an old GPU you could try that. Try to say your config, it might help a little bit more.

So you're saying the computer is running, but the screen/monitor isn't working? I had a similar situation after my cat jumped onto the computer case while I was playing a game. I’m not sure if it was the game, the cat, or just a coincidence. But in my case, disconnecting the computer from power and turning it back on helped.

I assume you don't have any spare parts to test your PC with? Otherwise there isn't much you can do.

I assume you already checked, but is the monitor properly connected? If the PC runs, you don't hear beep codes (which indicate some issues) and the screen remains black, my first guess would be the monitor cable is not properly plugged in or has some defect.