Inspiron 6000 Windows XP Stand by does not work

I've been using stand by for a long time now, but recently, when I close my Inspiron lid (it is set to standby on close), it will not go on stand by.  The drives and everything continue to operate at normal.  However, the monitor is black, and no matter what I do, it stays like that.  The activity LED light is flickers a little (mainly off, though).  Nothing works except to shut down the computer using the power button.

What is wrong, and how can I fix it (besides reinstalling XP)?

Thanks for all your help! :)

Notes:

 - I have not installed/updated any new drivers

 - SP 3

 - Pressing power button, clicking mouse, hitting keys do not cause the computer to resume.

 - Waiting does not work; I left it overnight and woke up with the same status.