You should certainly ensure the iPad has enough charge to restore properly. A fully exhausted battery will cause problems as the charging does not kick in until later in the boot process, so if there is not enough juice to get it there it will fail.
Connect the iPad to the WALL charger for a good few hours. If you don't have a wall charger, get the iPad into DFU mode and leave it connected to your PC overnight and for as long as possible (the PC will only trickle charge the iPad). Give it at least 8-12 hours. But the best bet is definitely the wall charger.
Connect to iTunes and hit the update button.
your device should be charged up before performing a restore. Most computers will not charge the iPad when connected to the computer.
also if you say you jail broke the iPad, there is a slight chance that something from the jailbreak process is causing iTunes on your computer not to properly do what it needs to do.
first think i would do is charge up the battery all the way and perform a restart. Apple has built these devices like tanks when it comes to this, its very hard to actually brick an iOS device. Having the battery die while updating firmware on any other device would pretty much render it unrecoverable.
If you still cannot restore with full battery, try performing a restore from a different computer.