Have you tried turning it off and turning it on again? (The computer, that is). Yes, I know that sounds facetious but it's amazing how often it works!
Has the monitor got an alternative input? If so, there might be a button (or a setting within an on-screen menu) which switches between different signal sources. For example, if the monitor is expecting a signal from a VGA connector, but you're using an HDMI one, it's going to show 'No signal'.
Make and model of the monitor,please?