You should mention your computer model and operating system and whether or not you can hear the computer running. Sounds like your computer could be sleeping or hibernating. Press the hardware button on the computer (not the monitor) briefly and see if it wakes up. If not, hold down the button until the computer turns off and wait 30 seconds and restart. If that doesn't work, you might have a video card or other failure. Of course you have rechecked all your cable connectors and diagnostic lights on the computer. Attaching another monitor would be a good test.

Most Dell flat panels provide a self-test feature that allows you to check whether your monitor is functioning properly. This is accomplished without connecting your monitor to the computer. The steps involved vary by model. Go to the link to find your documentation. http://support.ap.dell.com/support/systemsinfo/documentation.aspx?c=hk&l=en&s=gen&~cat=3