Problem waking PB screen from sleep

I have a 15" PB with os 10.3.5. I have begun leaving it on with the screen open, and have set the power settings to never sleep the actual computer when plugged in, but to sleep the screen after several minutes. A few times now, however, i will be prompted to put in the password, then the screen will go black like it's about to show my desktop, but nothing happens. The cursor stays a cursor (no beach ball). I was wondering if anyone knows what this problem is and how i may fix it.