What should I do if the PC constantly reboots, preventing me from entering the BIOS?

Question

Iron

This started happening after I turned on sleep mode.

Answer 1

That is, not that it is impossible to get to Windows, even to the BIOS - does it reboot?
The computer is broken and needs to be fixed.
As recommended above, look for the "guilty", start with the PSU, and then as standard - memory, vidyuha, discard disks, ...
There are no spare parts and a "donor" - to the service.

Answer 2

By chance, we are not talking about Windows 10?
If yes, then try this.

Answer 3

Agree with the above answers.
I had a similar situation six months ago. Turned off the computer, turned it on - an endless reboot.
As a result, the PSU died and took the motherboard with him to the next world :)
I was lucky at least I didn’t take the percentage. So if it’s the PSU, pray that the rest of the kit is alive!