Same thing will happen if you set the shredder too low and hit a bump of soil or tough grass clump--or hang a chain on something that won't budge.
If you've never before run a shredder on your place you might wind up inadvertently "landscaping" the bumps and clumps into dust until you get the machine set where you want it to run.
You didn't say if your machine has a slip-clutch or shear pin. If a clutch on the driveline you need to adjust and / or lube it. If a shear pin you might now find it partially sheared or just hanging on by the pin being 'smeared' between the mating sections.
In any case, after the hit you describe, it's time to change the pin and / or confirm the correct pin is used. You don't have to have a magic pin--you might use a Grade-2 (no markings on head) bolt long enough the slick shank sticks all the way through and a NyLock nut. Keep spares on hand.
If you have an inline clutch, after getting it working, keep the clutch covered if stored outside. I cut a 5-gal bucket to saddle down over it to keep the rain off. Watch for wasps later.
Another consideration: check your machine for bent blades, one or both. Bushhog blades can do and will bend and break.
You may also now need sharpening. You can use a hand grinder but be sure to support the cutter if you don't remove the blades.
Please post back your continuing experieces so we may all learn.
