Try flipping the little black switch - the side switch - above the volume rocker on the side of the iPad. If that doesn't work, double tap the home button and the task bar appears. Swipe all the way to the right in the task bar and look all the way on the left and there may be a lock icon there. Tap it to unlock the screen rotation.
You can set the side switch to lock the screen or mute system sounds. Go to Settings>General>Set Sidw Switch to: and select Lock Rotation or Mute System Sounds.