There are many possible explanations, unfortunately. If this happened at a particular time, e.g. after a call, then it would give us some clues.
What phone is it? Apart from rooting have you made any other modifications to the phone software? Did the problem start after you rooted, and if so was that straight away or later? Have you tried anything to fix it? Have you tried to update the software since it was rooted (not a good idea unless you return to stock first)? Have you checked that the proximity/light sensor isn't covered by dirt or a case?
Sorry, no answers because we'll need more information to identify the cause. Those are just a few questions that occurred to me.