[wanna_b_a_spore_modder]

Not only should you try to charge the battery by booting the PSP on AC power only, and no battery in, then put it in to see if it will charge, but then on top of that, you should check and make sure nothing is messing with the connectors of the battery, just blow some canned air in there. On top of that, like the others said, make sure you are holding it in tightly. It really sounds like the battery just discharged over the time that you weren't using it.