Compaq does not seen monitor

I just picked up a tower (Compaq Deskpro EN) and it was working at the center I purchased it from. Meanwhile, I transfered a monitor from the other unit that IS functioning fine, in fact I am using it right now. But when I try to boot up the machine to load the OS, the monitor is black…not DOS, just black. Any hints? This same monitor worked for Linspire and XP and 98SE. Why not for the blank Compaq?