A lock with a circle around it appeared in the right hand corner?
That means that rotation is locked. The screen will not reorient when you turn your iPad.
See this for information on how this works: http://support.apple.com/kb/HT4085?viewlocale=en_US&locale=en_US
There are two ways orientation can get locked. Most likely you inadvertently switched the lock on by sliding the little switch next to the volume rocker switch on the side of your iPad.
There is also a Setting that can be set to lock screen orientation.
HELLO ME! Thanks your reply jarred me back to reality!
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