The screen wont turn on

Hello i have changed backcover on a iphone 3gs and now it wont turn on ? i have done this a few times with succes but now it wont turn on when i plug it into the computer its make a beep and it come up on my computer screen but nothing on the lcd

i have maybe pluged out the lcd cable when the iphone was turn on cause i couldnt turn it off

any ideas what to do? replace the lcd???

sorry my bad english :-D