Please provide more detail according to following points:
- Macbook was off? sleep or shutdown? also what state had you left it in last time you used it?
- After it didn't accept your password, did you force shut down or shut down normal way?
- "won't power back up" what happens when you press the power button? the screen just stays blank, or you get some sort of display?
You could also try resetting your NVRAM by pressing (cmd-option-P-R) at the same time and pushing the power button. don't let go of these keys until you hear startup chime TWO times.
Lastly, try booting from a USB drive or DVD disk with Mac OS installed on it.
@ssdw if that's the case I would try to reset NVRAM and also SMC. The instructions for that are given on these sites
if that dosent work, then I'm afraid its a hardware problem likely to do with your logic board or display.