It shouldn't be negative unless you're cooling something. But even then, the question would ask for "energy lost" so it would expect a positive nubmer.
Always check if your answer is reasonable. In this case, you should be expecting something positive. Use your common sense - if it seems unlikely, check it by working backwards. But don't get too hung up over it (I've done that before) - sometimes unlikely-sounding answers turn out to be correct.