Cable to front drive system to loose

adjust the cable to the stop on the end of the bracket behind the carby body
remove the recoil starter and replace the spring

Is the tension spring gone? How is the cable? Usually there is a spring at the end of the cable in which when you pull back on the bar with your hands, this tightens the belt so that it engages with the engine allowing the wheels to turn.

Replace drive cable

Toro 20016 self propelled walk behind when the wheels are engaged it moves, when the wheels are not engaged, the wheels look up.
What should I do to fix the problem

Remove the left wheel and check the gears inside the wheel and the drive sheft. They might be worn or sheared and need renewing or maybe they just need oiling. Spray them with WD 40.

yes...tightening the belt happens when the handle is pulled...take the cover off the belt an see if there is play in the pulley on top of the transmission...if there is the trans is bad and must be replaced...

Is it possible that the belt has either broken or come off the pulleys? Next step would be to remove the cover and see if there is something preventing engagement or if you see any broken parts. May even be as simple as lubricating the cable/levers.

Disassemble and, re-engage the pull cord locking pawl spring

If you take the wheels off and remove the belt from the small pulley on the the front axle underneath the pulley you should see a small transmission. You usually can split this unit in half . You will see two gears and sometimes one plastic and one metal. The plastic one is usually the culprit. Hope this will help you out. Donnyb60

On my Toro recycler, I also had the front drive stop working -- and fixed it after reading the solutions here.

First, the plastic cover on the top deck that goes over the front drive mechanism must be loosened enough to pull it up. Before you can do that, on the underside there is one screw to take out.

Once that's done, all you have to do is pry the plastic cover up enough to see the belt and pulley that make up the front drive assembly. On mine, the belt had simply come off the pulley. Putting the belt back on, then tightening things back up is all it took to put the Toro back in sel-propelled happiness. Good luck!