Will not power on with a full battery

Hey everyone, first off I would like to thank each and everyone of you that dedicate your time & Knowlage to us... So my problem is that my little brother had a first gen PSP and he didn't take the best care of it.. So he lost it one day and I found it stuck in my couch that my family gave me when I moved out. I gave it back to him but he didn't want it because it wouldn't power on. I tryed charging it and there is no life in this thing. I even took out my battery that is fully charged and still, nothing...?

What could be the problem? And what guild will u follow?

I wanna fix ir for him and surprise him with his old "working" PSP!

Because he really loved that thing =)

Any help or shared thoughts will be great. Thanks again!