PC suddenly no longer recognizes second monitor?

I have a dual monitor setup, which I've never had any problems with. When I turned on my PC earlier, I noticed that one monitor no longer had a signal. I unplugged and replugged everything, etc., and then realized it wasn't a problem with the monitor (both worked), but rather a problem with the PC because I was suddenly only getting a signal on one monitor. I can now only use the monitors individually, but when I try to use both at the same time, one no longer receives a signal. Does anyone know what could be causing this?