Depending on how low of a temp it may not be that bad. To start, the internal temp on your fermenter will end up being few degrees warmer than the room due to the activity going on.
If you are really concerned, check the manufacturer and see what the optimal ranges are for the yeast you pitched (wyeast provides this info on their website).
If measures need to be take, you can move your beer to a warmer room, wrap it in blankets, rub it (OK, that one was a joke...)...
If anything too cool is better than too warm. You may take longer to fully ferment but you won't get any off flavors.