Unable to awake from sleep mode

This weekend I turned on my PowerBook and walked away while it was booting. I have two different login account set up so you are required to select an account and enter a password. I must have walked away longer than I thought because the PowerBook went into sleep mode at the login screen. No matter what I tried I couldn't get the PowerBook to wake up. Is there a special key combination or something to wake it up when it goes into sleep mode at the login screen. This has happened to me before and the only way I was able to wake it up was to connect it to an external power source. I would have done this but I had left my power adapter at work. Any help will be appreciated.

When was the last time you charged the battery? If it is only coming out of sleep when plugged into a power source, it is possible that your battery is drained.

The battery was down to about 50% or so.

I've hit the spacebar sometimes and that seems to work. Which I just recently switched, so there may be a key combination that I am not aware of. Sometimes I just find myself closing the lid back down and re-opening it to input my password.

Try hitting the spacebar and see if that helps.