Question PC goes up till motherboard's screen then enters a boot loop. Shortly after that, there is no signal in all of the I/O devices

Aug 3, 2019
So two weeks ago I shut down the PC normally and went to sleep. The next day it won't go beyond the screen which says "Press F2 to enter setup" and will just loop (turns off then on again off and so on). I turned off the system py pressing the power button for a few secs. The next time I start it won't display anything and the keyboard and mouse won't show any life. All the fans (PSU, CPU and Video Card) are spinning and power LED, HDD activity LED and Optical Drive LED are lighting up. I don't have a case speaker so no beep codes. I tried everything
I could do which was posted on the "post-no-video" troubleshooting steps provided here but nothing worked.

Took it to a nearby repair shop. The PSU, Motherboard and CPU had shorted out and I had to replace the PSU and CPU. After the repairs, they turned on the mobo and it worked alright till bios screen. Then they fixed it on to the case with all accessories like HDD and Optical Drive. Then when they turned it on, it booted properly till Ubuntu's Login screen.

When I took it back home, the same problem persists. I'm 100% sure the monitor and mouse are OK since i tested with a laptop. Can't say about keyboard since it's old with those pin type cables.

Could it be due to the UPS? It's over 8 years old and gives enough backup to save stuffs and shut down. It never died by itself. I'm afraid I've to replace the parts again. Please help me out!