The quick test for a monitor is to use on another system and check results. If it appears OK, then you have a system or video port/card problem, if not then bad monitor, replace. You should at least try with it unplugged and if capable use its OSD display(monitor setup), does it have one? If not, then basically if no display and trying the above, its a goner. You did mention "old monitor" and if its that old and you tweeked for best display on new system, you may have driven too far, though capable its age caught-up with it. Reseat cables as well to check that out. A decent new LCD on sale maybe a bargain, so check online oir local stores for such.
Just bought a new HP Pavilion Elite HPE-112y Desktop PC//but running my OLD gateway FPD1730 monitor. All worked fine for about 1 month - now it has gone black but the power button is still on.... Any kind of feedback other than purchase a new monitor.. Either way - thanks for help!