iPhone 4 - Will turn on plugged in USB/Wall, but not charging

Hi All

Please help!

I recently upgraded to an iPhone 4S and as such decided to sell my reliable iPhone 4.

However, after selling it, i got email from the buyer advising that it crashed afer 6 hours and then would not turn back on or charge.

I was forced to take the phone back and am not sure if he hs tampered with it, or changed parts etc.

I have attempted a restore, but it says Unknown Error 21.

I have 2 questions:

1. Can this be fixed? Any ideas would be great as i am stuck!

2. Is there a way to confirm the IMEI Number of the phone without it turning on?

Any help would be great!

Thanks all