While I’ve only used the Galaxy Tab a couple of times, I am apparently the person of choice to fix a frozen tab among friends.
To restart, hold the power button for about 15 seconds. You should try this first if the device is frozen. Sometimes it won’t respond to the first attempt apparently, so try a few times. A restart is basically cutting power and turning the system back on. It should work for most frozen cases.
But then there’s the next level, a complete crash. I don’t know what would cause this to happen, but I’m sure it does. It’s the last measure as it will erase everything from the system you have customized and replace it with a factory default. To reset, hold the volume up rocker and the power button for about 15 seconds. This will initiate setup sequence guiding you through the motions of the first time you setup the device. This said, it will not erase any SD card contents, so pictures, apps and other data stored on the card will be left intact.
The Galaxy Tab’s restart method will probably be the same regardless of carrier, so a AT&T Galaxy Tab will use these same steps as the Wifi Galaxy Tab.