Start drive gear isn't engaging the flywheel...the battery might be bad, check the voltage, and then while you have the key in start and starter spinning, check the battery voltage, if it's dropped below 10 or so volts, probably a bad battery.
also, most auto parts stores will load test a battery for FREE.
if the battery check out, check the ground and positive connections, be sure they're tight, at the battery, on the starter, and solenoid, and where the ground wire grounds to the frame, Might even take them off and wire brush them if they're dirty or corroding.