Most likely the battery. They will simply die on you and leave you stranded sometimes, or perhaps the cable is corroded or loose. Since yo have no power anywhere else, not much else it coul be. Of course if your alternator was failing all along, you could simply have been running the car on battery and now it is dead. Look at the terminals and clean if need be. If it still does nothing have someone help you and take the battery to local Advance auto or Autozone or NAPA store. They can test to see if the battery is simply drained or bad. If it is simply drained you will need to have someone charge it back. Then, install and see what happens. If it fires up get to same place QUICK and have it tested for alternator strength. If weak you will have to replace it. Not a difficult job, but be careful anytime you are dealing with batteries.