Its a Dieffenbachia. Over-watering and under-watering look exactly alike because the results are the same: dead roots. Try to find out what kind of watering or non-watering your housemates might have done.
I would start by taking the smaller pot out of the larger pot to check if your plant has been floating around in water. Over-watering Dieffenbachia is more serious than under-watering. Feel the soil. It should be dry about an inch down before you water. AND the plant should never sit in water.