Your iPad's contents are backed-up on your iMac, right? Hopefully you didn't attempt a restore without having first backed-up.
Check your user's home folder->music folder (the default location for applications and files managed by iTunes) to confirm your iPad's contents are there. Otherwise, it sounds like you may need to cancel the restore (which hopefully reverts the iPad to it's previous state) and reattempt once you have sufficient storage for your iMac.