Does it make any difference if you press the D key then try again? If your canvas has been accidentally dragged that would reset it to its original position.
Does starting a new painting have the same problem? Also, try restarting ArtRage, then, if you haven't already, restarting Windows just in case there's a driver problem of some sort.
I assume it was working ok previous to this? If so, if you can think of anything which changed or was different just before you started to see the problem, that may be useful ( don't worry about that part if nothing springs to mind ).
Would it be possible to post a screenshot of the problem for us to look at?
You can see how to post an image here - http://www2.ambientdesign.com/forums/viewtopic.php?t=12
Also, can you let us know what system you're using and what make and model of tablet you have?