|you may have loosened up a connection. reseat everything. Canned air shouldn't hurt anything. You did get everything out? |
Maybe a wire is rubbing on a fan. That can stop it from running.
Worst possible thing you may have done is dislodged the heatsink. In which case you may need to remove it, clean off thermal paste, reapply and reinstall.
Don't attempt that now. You need to know how to remove it or you can damage things. I bet it is just a loose connection. push them it all the way.
After rereading your post I think you may have the monitor connected to the wrong port. Look to see if there is more than one place or maybe even an added in video card that you are NOT connecting to.