iPad not reacting to screen orientation - screen lock
Understanding The Screen Rotation Lock
Apple has a support document that explains this called &quot;iPad: Understanding the side switch&quot;.
Here's a quick explanation:
If your iPad has version 4.2.1 or later of iOS installed (Settings &gt; General &gt; About &gt; Version) there are two possible ways your screen can be locked.
The default is the software lock. To adjust this rapidly click the home button twice. This will reveal a bar below your dock. Swipe the icons that are revealed once to the left to reveal some additional controls. Your screen should look like this:

You can also enable a hardware lock. This will cause the small switch on the side of your iPad to lock and unlock screen rotation. The setting for this is found in the settings apps here:

If this setting is set to &quot;Lock Rotation&quot; instead of &quot;Mute&quot; the software lock will be replaced with a mute button like this: