The starter solenoid is not energizing to push the gear forward to engage the flywheel starter ring. Check your starter wiring for looseness. You can just replace the solenoid if you want although a complete starter replacement may indeed be needed.
It could be the Bendix clutch on the starter drive too. Solenoids are about 28 bucks and a drive is about 18. No matter which way you go if you hit it wrong the first time you end up with a new solenoid and new drive on an old starter motor. If you can get a rebuillt for 50 you're way ahead and you only did the job once.