If you can go forward but not reverse, let’s be sure we are on the same page with this problem. Will the gear lever move into reverse? Or does the gear lever not go all the way into reverse?
If it goes into reverse, you may have a transmission problem that needs inspected. If it doesn’t go all the way into reverse, check to see if there is something such as a stick or debris at the transmission shift shaft that is blocking it.
Debris can cause the shaft to not move in a certain direction or block it from its normal travel. If you cannot find anything that is causing it to not go into reverse, this would be a good point to take your mower to a repair shop for an evaluation. You can visit our locations page here to search for a lawn mower repair shop near you.
Sometimes the shifting parts get bent or seize up with rust. A bolt can become sheared or a pin come loose. There are many different things that will cause this issue and a good lawn mower professional will find the issue and resolve it.