Sounds like the PTO over-running clutch has failed. The PTO OR clutch is not really meant to be a safety or a slip clutch, it is meant to function when using a mower mostly. Without an OR clutch, the mower will power the transmission and drive the tractor forward even if the foot clutch is depressed. It is dangerous. Old Ford 9Ns were this way, as were most of the early grey market tractors. For those type of tractors there is an OR clutch adapter that goes on the PTO and keeps the spinning implement from driving the tractor. Modern tractors have this feature built in. Earlier OR clutches were ratcheting types. On a 2615/2816/3
015 if you push in the clutch with a spinning mower, you will hear a loud clicking ratcheting sound. Noisy, but bullet proof. So that is some history of OR clutches.
The 2415/2516/3016/3616 uses a silent OR clutch. They work fantastic, but if you get into a rototilling situation where the tiller is bouncing off rocks it will cause the OR clutch to slip. It is not designed to slip. If you do this enough times, it causes the OR clutch to fail. A tiller really must have a slip clutch of its own, and it needs to be adjusted so it slips before the internal OR clutch slips. If your tiller has a slip clutch and it is like most slip clutches, it is stuck. They need to occasionally be loosened up, slipped a little to clean off the rust, and then tightened back down correctly.
The tractor has to be split to replace the OR clutch. It is a simple enough job for a trained technician. Nothing complicated, but it does take some time.