Cranks but won't start

Initial questions, how old is the battery and how long has it been since you've checked the battery? I would say most likely the alternator is not working anymore. A cars alternator can last anywhere from 4-8 years depending on how well you use your car's A/C, Heater, and other electrical devices while driving the car. It is a part that inevitability needs to be replaced. It would be beneficial to also have the car battery checked because it may have been drained, and questionable for future use.