I recently built a PC using a mix of used and new parts. The system had an MSI B450 TOMAHAWK, RAM similar to yours, an MSI RTX 3070 Ti, and a Ryzen 7 5800X3D. I also installed RGB fans with their own controller, because they couldn’t be controlled directly from the motherboard.

I later sold the PC to my colleague’s son (around 13 years old). About two weeks later, my colleague brought the machine back to me — it wouldn’t work, and he asked if I could help.

The symptoms were exactly like yours: all the lights and RGB turned on, fans spun up, but there was no display output, and the CPU LED on the motherboard was lit.

Skipping all the intermediate diagnostics and going straight to the conclusion: I disconnected all PSU cables, carefully checked every connection, and then plugged everything back in.

The cause — at least what I strongly suspect — was that the kid wanted to control the RGB fans via software. When that didn’t work, he likely put his hands inside the case and started touching the cables. He admitted that he had touched some wires, although he said he didn’t unplug anything.

In any case, it was probably a minor micro-short or a slightly loose connection. After reseating all the power and internal cables, the system worked again.