No signal message

Can outside interference cause no signal message? I have a computer at school that had been working fine at the beginning of the year and now we are having an issue with the no signal message. A couple of months ago, we had a signal booster installed in one of our classrooms and now the monitor is not getting the signal. Could this booster be the cause of the problem. The booster is in the same area as the computer, but there are computers on either side that are working fine. I even tried changing cables, monitors, and even a new cpu. Nothing has worked. Please help!