It's likely that you are not getting enough power from the battery to hold the starter solenoid in to start it. Check the voltage on the battery while you're trying to start it. If it's 12.6 (more or less) then you have a bad connection somewhere. If the voltage goes down (way down), then you have no charge in the battery. Charge it, check that the alternator is charging (13 - 14 volts) while the engine is running with a few rpm's. If not, replace the battery.
Jim
Jim