Steam needs to be online to update

try turn off your firewall

did you look for it on google?

If you tried what ndx8 suggested and it didn't work, you can try removing everything except steam.exe, /steamapps (and /userdata if you have screenshots that aren't saved to the cloud), then reboot your computer, powercycle your modem/router, and then try launching steam.exe (as admin)

Try starting it on Administrator mode.

Task manager and then you end all tasks relating to steam. Then run it again.

Hack Steam Server so they don't update anymore so you won't have this problem in the future.
Bump

Maybe it's an antivirus software. Look here: https://support.steampowered.com/kb_article.php?ref=7333-UAXB-2919

https://steamstat.us/ -
"Steam Store and Steam Community are down"

maybe this is part of the problem.

Have you done a malware scan with something reliable, like Malwarebytes' Anti-Malware (MBAM)?

I had the very same problem on my computer, but it didn't happen on the connection from my university (which is supposed to block steam access!).

Go to your router settings, enable DMZ mode for your ip, then disable it.
I know, it doesn't make any sense, but that's what I had to do and steam support didn't help.
I hope it works, good luck.