you have to restore it through iTunes... No harm done, you'll just have to re-sync all of your stuff, and re-jailbreak if you've done so already. basically, your Touch will be like it was out of the box, no worries.
Apple iPod Touch, 16 GB
30 GB Microsoft Zune (alive and kicking)
5G 30 GB iPod Video (stolen)
San Disk Sansa mP3 player, 512 MB (DEAD)
San Disk Generic mP3 player m230, 512 MB (Alive and Kicking)