My desktop PC won't start and I can't access the BIOS.

Hello,

I hope you are doing well.

First, try removing your RAM sticks and putting them back as they were. Ignoring the CPU LED, this is generally a RAM-related issue.

If the CPU LED lights up at that location, according to your motherboard manual, it means that the CPU is not detected or that it is faulty. Honestly, a CPU failing overnight is extremely rare. Here’s what I would suspect:

• A motherboard issue (perhaps with the VRMs, which are the voltage regulators).
• A power supply issue (even if everything seems to be working).
• A BIOS issue: try doing a "clear CMOS" (this will reset your motherboard to its factory settings, or defaults). This solution has helped me more than once, here’s how to do it with your motherboard:

• Turn off the computer and unplug the power cord.
• Use a jumper to connect the JBAT1 pins for 5 to 10 seconds (see the location on the diagram).
• Remove the jumper from JBAT1.
• Plug the power cord back in and turn the computer on.

You can also use a screwdriver to create a temporary bridge between the two pins, the PC being obviously turned off.

Hopefully this helps, let me know if it does. :)

Have a great day!

Hello and thank you so much for your really appreciated help in such a situation.

1) I could only remove two memory sticks out of the four; the remaining two are stuck behind the cooler. I cleaned the contacts, but there was no sign of dirt or oxidation on the two I could access.

2) I performed the "clear CMOS" using a screwdriver since I didn't have a jumper.
I'm not sure if the operation worked, but there’s no change to report. The CPU LED lights up immediately in solid red, no signal reaches the screen, and there are no beeps at startup as usual.

I checked all the (accessible) connections on the motherboard and also those behind the Corsair power supply. Nothing is unplugged; everything seems to be in place.

I had thought about using a Windows 10 installation USB drive, not to reinstall the system but to try to access "troubleshooting" mode.
But since I can't access the BIOS, I assume I can't boot from the USB drive?

I’m open to your suggestions. It's a shame not to have access to run diagnostic software...

Have a good end of the weekend and thanks again!

The day before, the PC was working normally; it was the next morning that it wouldn't start. I haven't installed anything special, but I use it intensively for photo editing (Luminar Neo, Lightroom, etc.), and the PC is under a lot of strain.

I chose the Ryzen 9 3900, which runs cooler than the 3900 X, because I live in Thailand. The processor is cooled by a large heatsink with two fans, and the case has 7 fans.

Do you think it could be the NVME SSD that is faulty? I imagine that wouldn't block at the CPU LED level?

I'm going to take the PC to a workshop, and I will get back to you to let you know where the problem came from, hoping it's not the CPU that has given up the ghost…

Thank you again for your help, and have a great day.

Hello,

A defective hard drive does not prevent the PC from starting but does block access to the BIOS. Try disconnecting your hard drives and see if you regain access to the BIOS.

Hello, here's the end of the story. The verdict is that the Corsair RM 650 power supply was defective even though the motherboard and all the fans appeared to be well powered. The motherboard was also damaged, so I had to replace both the power supply and the motherboard. However, it wasn't easy here in Thailand to find an AM4 ATX motherboard. I had to take a GIGABYTE AORUS ELITE instead of the original MSI.
Thanks to the contributors of this thread for their help.
Best regards. Michel.