Are you plugging it into the same jack on the computer that the CRT was in?
Is it a VGA connector or a DVI connector or are you using an adapter for the LCD's cable that you were not using with the CRT?
Some things to try...
Check the connection of the cable from the computer to the LCD monitor. Are the plugs plugged in correctly? Does the monitor have more than one channel (mine is old but has 2 channels on it)?
It displays a "no signal" error so the monitor appears to be functioning. The CRT worked so the computer appears to be functioning
It's probably something to do with the computer to monitor connection then.