Monitor won't stay on

This is probably very simple, but I can't get anything to work. My father
took out the box that holds the expansion card slots and we installed a new
network card. The built in one didn't seem to work when he tried to set up
cable internet. When he took it home, the monitor wouldn't stay powered on.
He tried switching monitors, same problem. I noticed there was a built-in and
an expansion card. I tried both, unplugging all cords and plugging back in.
Still no luck. Tried reseating the box. No luck. I can't remember if it's a
Compaq or HP model.
--
aly