Question PC won't start properly - Blackscreen with underscore in top left corner ?

[iblybol]

When I went to turn on my PC this morning I was immediately met with a black screen and a white underscore in the top left of the screen and keyboard and mouse did not even switch on. I last used the PC the night before where absolutely nothing was wrong and I did not change anything.

I have so far tried the classic turn it off and on again approach (multiple times) and I have also tried using a memory stick to flash BIOS which did not solve the issue. I have owned this PC for a few years now and not sure why this has suddenly happened. Has anyone experienced this before/ can provide any help it would be much appreciated!

[UPDATE] I removed the CMOS battery for 5 minutes and put it back again and this fixed the issue temporarily however after booting up the PC today I am faced with the same issue and removing the CMOS battery no longer fixes it. I have also tried using a new CMOS battery and this did not help either. If anyone could help at all it would be great because I am completely stumped and would rather not have to buy a new motherboard
Thanks

PC specs:
B450 tomahawk max motherboard
AMD Radeon 580x
AMD Ryzen 5600 processor
Corsair CS550M power supply
2x Corsair Vengeance LPX 8GB RAM
1tb harddrive and 500gb harddrive (unsure about the specifics of these)

 

[iblybol]

I have now tried removing each RAM stick, removing GPU, unplugging both harddrives and this has not helped...
I feel like I have actually made the issue worse because now when I switch the PC on it only breifly flashes a dark red and then immediately loses signal again :/