OK - there are several possible reasons for this - from simple to more complex. So, let's try a real simple one first of all - and, simple as it is, it fixes about 80% of unexplained problems.
Switch the iPad completely off - i.e. hold down the Power button until the slider control appears, accept the offer to switch off, wait until the rotating white bezel disappears and the screen is completely blank, then switch on again by pressing and holding the Power button until the white Apple logo appears. After a few seconds the iPad will play a short 'jingle' and, a couple of seconds later, you're back to normal. Surprising as it seems, this fixes a whole load of problems and even Apple recommend you do this occasionally rather than simply putting your iPad to sleep - as most of us normally do - with a brief press to the Power button.
If this doesn't fix the problem, there are a load of other things you can try so get back to us and we'll suggest some more
Tim
