This old SS20 won't boot!

Recently, I was given two SS20's that I discovered won't boot, even
though they worked perfectly well two years ago. Funny thing though, all
of their parts work- I have another SS20 server that I used to test the
CPU's, the graphics cards and the NVRAM chips (the ones with the yellow
sticker on them.)

When I turn on the machines, it lights up, the hard drives spin up, but
the monitor stays dark. What could I be missing?

Hopefully someone has an idea of what could be wrong or else I'll have a
bunch of parts to sell on eBay...