When your battery gets low enough that it shuts down on it's own it often wont boot up, even while charging. It simply doesn't have enough power stored in the battery to complete the boot cycle. Let it charge for 1/2 an hour (it will keep trying to boot) and eventually it will get charged enough to start up.
well, something wierd just happened, I stuck it on the wall charger instead of on the computer, and it booted up, but it was on the screen saying I needed to connect to itunes to activate, and the odd thing was the battery indicator said it was completely charged but the second I take it off the charger it shuts down. I've tried leaving it on the charger, but i guess since its says its at 100% its not charging. I think the battery may have shorted or something.