No input signal suggests a loose cable or connection on the motherboard. Viruses wouldn't affect this. When you attach the cable to the back of your computer, do you have little screws to tighten the cable? If so, then detatch the cable and reconnect and screw-in the little screws. It is also possible that one on the pins inside the connection on the PC has been bent or broken but this is rare. Make sure the other end of the cable is connected correctly to your monitor. Some monitors have a few connectors, so make sure you are using the right one.
