You've probably got the slide switch on the side of your iPad (next to the volume control) set to the "locked screen" position. Move this slide switch to the oposite position that it is in now.
NOTE: This slide switch can be selected to either Mute the Audio OR Lock the screen rotation. This is done by going into your ipad settings. This switch can sometimes be unintentionaly moved from the off position to the on position as you are using your iPad.