A couple of things you could try here.
First, do a four finger swipe up which will reveal the task bar with all recent apps in it, touch and hold the app (is it drop box?) and a small - sign will appear, press this to close the app, then four finger swipe down to close the bar and re-open the app, if that hasn't cleared the problem, try this....
Reset your iPad by holding the power off button and the home button simultaneously, keep holding them in and you may see the power off slide appear and disappear, wait until you see the Apple logo appear before releasing the buttons. It will take a little longer for your iPad to start.
When it restarts see if this has cleared the problem.
Let us know how you get on?