PC won't boot after changing boot mode to legacy

Hi, this morning my PC completely stopped working and won't boot up. It powers on (all lights work, keyboard turns on, ect) but loads to a completely black screen.

The model is a Dell Optiplex 7010. It previously worked fine, but I've never been able to get BIOS to load on it, however I've been able to get into the boot mode menu by pressing f12. 

After switching harddrives recently, my PC automatically switched it's boot order, and tried to boot with IPv4/IPv6, before finally booting from my HDD whenever the ethernet cable was plugged in. I wasn't able to fix this issue, as like I previously said BIOS just wouldn't load on my pc (would always result in a black screen). so I tried changing the boot mood from UEFI to Legacy. after doing this my pc will now turn on, but only to a completely black screen. BIOS still won't load, I can't get into the boot mode menu either. if I press f12 on startup, some yellow text flashes for a single frame in the top right corner but the screen stays black. I've tried switch RAM slots, switching hard drives, switching monitor, Resetting the CMOS battery but nothing will work and I am completely stuck with what to do.

Any help would be greatly appreciated as I really can't figure this out.

thanks, Joe