Could be the throttle cable broke, came unhooked, carburetor innards problems. Those things wait out in the shed, and after you turn out the lights, they gather their strength, and fall apart! Been there, seen that! Ohhh, yes!

A broken governor can do that. How fast was it going? Much more than normal?

Check that governor arm has not come loose from governor shaft coming out the side of the engine OR some other linkage broken or off. Check that carb throttle butterfly has not come loose inside carb.

IF both of these are OK then probably governor has disintegrated inside engine. The governor shaft sometimes breaks on these.

Walt Conner