First try just force-closing the game: from the home screen, double-tap the home button, locate the game in the list at the bottom, tap and hold it until it wiggles, tap to close it, tap the home button again. Then see if your game works now.
If it doesn't, back up your iPad to iCloud, then restore your iPad to this backup. When you restore the backup it will restore your app data and other data and settings, then redownload purchased apps and media from the iTunes and App stores, assuming they're still available.
To do this, start by saving your photo stream photos to your camera roll (if they aren't already there) by tapping Edit, tap all the photos, tap Share, tap Save to Camera Roll. Then back up to iCloud (Settings>iCloud>Storage & Backup>Back Up Now). Confirm that the backup was successfully completed (look for the indication under the Back Up Now button), then go to Settings>General>Reset, tap Erase All Content and Settings, go through the setup screens on your iPad and when given the option, choose Restore to iCloud Backup. Sign into your iCloud account and select the backup you made earlier to restore to.