Do you mean that from "cold" the computer lights up and makes the familiar noises but the monitor doesn't display anything at all?
If this is what happens and there are no warning bleeps from your computer it is likely that something has happened to the video card or the cable connection.
When there is a loose video card, the "handshake" between the computer and monitor does not occur and there will be a pattern of warning bleeps. ......... But if no signal arrives from the computer, then the monitor will go to standby.
Can you find a simple old card to substitute as a test?