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 - here is a HowTo video on youtube

good luck