Double tap the home button and swipe all the way the right in the task bar at the bottom - see if the lock icon is in the far left corner of the task bar. If so - tap it to unlock the rotation.
Or check the side side switch above the volume control on the right side of the iPad.
You might want to read this as well.
If those don't work - try a reset.
Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.