If you see a lock icon in the upper right corner of the screen - then the screen orientation is locked.
Try the side switch above the volume rocker first and see if that unlocks the screen. If that doesn't do it, double tap the home button and swipe to the right and look for the lock icon all the way to the left.
If the screen is unlocked but still will not rotate, reset the iPad.
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 button
If it's next to the battery indicator and it looks like a padlock with a circular arrow around then you have rotation lock 'on' i.e. the screen doesn't rotate if you turn the iPad. Depending on what you've got Settings > General > Use Side Switch To set to (rotation lock or mute notifications), then you can lock/unlock the rotation by the switch on the right hand side of the iPad above the volume switch, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085