GX620 won't boot - at all

I have a Dell GX620 MT that I bought refurbished (I know, second mistake after buying a Dell--it was cheap, what can I say?). It was working fine until one day last week when I tried to start it up and it wouldn't. At first, I got the Dell screen with the progress bar, which was frozen at about 10%, then after that, attempts to reboot produced only a blank screen, with a little box in the upper left corner of the monitor switching between "Analog" and "Digital", like it couldn't figure out what kind of monitor was attached.

I figured it was likely a problem caused by the onboard video controller. I tried installing another video card, but no luck.

The power light is a steady amber (and so is the Aux Pwr light in the inside of the case), and I can hear the fan when it's turned on, so I'm assuming it's not the power supply itself, but if anyone knows differently or has any advice, I'm all ears.

TIA