|Make sure monitor light is on (at least amber) or it might be turned off or power cord may not be fully seated.|
Look over monitor cord (signal) to make sure it is fully seated at both ends and there is no damage to the cord or connectors.
If the machine was moved recently, check all internal an external connectors to make sure nothing is loose (power unplugged first).
If the machine was just put back into service or monitor or other components were moved or unplugged, make sure all is connected correctly (Some machines have more than one monitor connector, especially one on motherboard and one or more on a video card, make sure you are choosing the correct one).
Use a can of compressed air to blow out dust from internal areas, especially heat sinks, fans, and vent openings. Do not allow the compressed air to force fans to spin faster than is normal for them (use short bursts or hold them from turning).
You have to be a little bit crazy to keep you from going insane.