When computer goes to sleep, cannot turn display back on

Whenever I leave my computer alone for awhile and it goes to sleep, or I close the screen, when I try to turn it back on again the display will not turn on.  All of the blue lights will be on, but no matter what I press or try to do the display remains black.  I usually just have to hold down the power button and shut off the computer then it will restart.  Is there any reason why it is doing this?  Thanks