screen not working, but it is intact.

Hello fellow tinkerers...

i have a problem with my iPhone. have bought the thing off Ebay, described broken glass. ok. got it home, pretty good condition. screen working, everything fine. then i took it apart...

well, screen stopped working, but if i plug it into different 3Gs, it works...

also, if i plug working screen from my other 3Gs, it doesnt show anything.

Now, could the board inside somehow got damaged by taking it apart, or am i just having a bad day?

in that case, if i replace the glass, would those geniusses at apple store replace the phone? it still is in warranty...

thanks for suggestions