Powerbook won't start up

Hi - it powered down after running out of juice and not being plugged in at my desk. I plugged it in and started it up - it makes the Mac Bong sound and then the apple appears. But then a progress bar appears on the bottom of the screen and it doesn't even reach 1/4 of the way before it shuts down again.

I have tried it plugged in, unplugged - no difference.

Please help.

Thank you!

Bill