A number of users have reported an issue in which all videos, including movies purchased from the iTunes store and other media, are missing after a sync with iTunes. Oddly, some users have reported that the videos return after repeated sync attempts.
Sync with nothing, then everything. In iTunes, select your iPad from the left-hand pane, then deselect every sync option available (movies, music, etc.) and sync the device. After the sync is complete, reselect all desired items then sync again. This may restore access to videos on the iPad.