First: If the battery is more than two years old, it could be the battery. They are good for two years, plus several more starts, then they do a belly-flop, and you have to buy a new one!
Second: If your engine is the over-head valve type, have the valves adjusted. Valves out of adjustment can cause your problem.
Third: Remove the engine flywheel shroud, and remove any mouse nests under it, and clean out the spaces between the fins. Blow out anything in that area with compressed air.
Make sure the switch that turns on the mower blades is in the "off" position.
Fourth: Check and clean all battery connections, including where the negative (-) cable attaches to the frame, or wherever.
Fifth. Look for the little flat fuse holder, should be near the starter solenoid area. Look for a blown fuse.
If all else fails, seek professional help. DO NOT buy anything, unless a professional fixer has first inspected it! HTH: Rusty Jones

I have the same mower and had the same problem. I was determined not to buy a PTO. I don't believe things just stop working suddenly. (usually)
My fix was to locate the wire going to the PTO. It is on the left side, sitting down, on the frame next to the brake pedal. When I pulled the wire through the access hole,I noticed the connector was not pushed in all the way. Pushed it in and all was well. I love saving money.
I can't tremember if the starting problem was related to this or not but it was fixed by removing the connection under the hood to the ignition and hooking it up again. To this day I still have to jiggle the connector from time to time.I hope this helps. If not at least we killed three minutes.