Mac Mini Doesn't boot but HDD works

Hi everyone. Hoping to get some answers from the community. We're currently using Mac Minis (2011). I stood up for a bit while I was in the middle of a photo editing session, when I sat down again the screen was all vertical lines that were blue-green in color. Moving the mouse or typing on the keyboard didn't snap it out of this so I did a hard reset.

It powers on, Apple chime and logo come out, then the logo goes away and leaves me on an empty grey screen. No crazy beeps or flashing lights or any other signs of malfunction, but the OS (10.7.5) doesn't complete booting.

I tried booting up from an external HDD clone without success. Using the other Mac Mini however, and using Target mode to allow the broken one to be read as an external HDD, I was able to boot up from the HDD of the said non-booting Mac. I'm inclined to think it's a hardware problem.

Any thoughts on what exactly the problem is on this thing?