Computer not recognizing monitor

Awhile back I installed a new stick of RAM into my computer. My computer then stopped displaying on my monitor. After fiddling with my machine for awhile, the computer suddenly started working perfectly, even with the new RAM installed. Then I went on vacation for a couple weeks, got back, and my computer again refused to display on the monitor. I’ve now tested a different computer on that monitor and different monitors on that computer. Everything seems to be working right, but nothing is displaying from that computer. I removed the RAM just in case, and I know power is getting to the video card, because it has a fan that switches on. Any ideas on something I haven’t tried?