What did you use to try and jailbreak it? Was it interrupted?
Put it into Recovery mode or DFU mode and restore and try again or with something else.
In case you don't know:
Recovery mode -
1. Turn it off
2. Hold the Home button until the recovery icon (iTunes symbol with a wire going to it) appears
DFU -
1. Turn it off
2. Hold the Home and Power buttons for 10 seconds (exactly)
3. Release the Power button but hold the Home button until it is detected by your computer