what iphone do you have ?
i would first try to reset it - press and hold the sleep and the home button at the same time until the iphone restarts - if that won't help - connect it with the computer and start itunes, does itunes show any error ?
if the isn't recognized by itunes, put it in dfu mode and restore it
if you don't know how to put it in dfu mode - [http://www.youtube.com/watch?v=bITIiGswjFI|here is a HowTo video on youtube]
good luck