I have a W10 system that has been running for more than 3 years. Case fan went bad, so I replaced it.
It will not boot, I get a Gigabyte splash screen then the UEFI Gigabyte banner announcing that it features a "Dual BIOS.," then the screen goes black, repeat, repeat,repeat. That whole process takes about 15 seconds.
I can't get into the BIOS, I pressed Delete many times,
I have removed the battery to reset the CMOS. I have removed all cards except the video card. I have found no loose cables. All RAM skots are full and all are firmly down,