Being an electronic/computer type device that can happen and often a Simple Restart or Reset (as described below) will often cure many such ills.... If the problem persists... Connect to iTunes on your Computer and Restore
Reset ( No Data will be Lost )
Press and Hold the Sleep/Wake Button and the Home Button at the Same Time...
Wait for the Apple logo to Appear...
Usually takes about 15 - 20 Seconds... ( But can take Longer...)
Release the Buttons...