Try removing the battery cable connections on both battery terminals and clean them well. I've experienced similar symptoms due to corrosion between the cable connector and the battery post (or solenoid, or ground). If this happens, its possible nothing electrical works: starter, lights, etc. It can start and operate fine one moment; then the next time, nothing. If you have a voltmeter, compare the voltage measuring between the battery posts, then from the cable connector to the other connector, without touching the battery post. Good luck.