First, I recommend you disconnect the spark plug, then lift the front and have a helper stand on the handle to keep it up. Inspect the blade for being badly bent on the outer end of one portion (it will be obvious if it is). If it is, not only is the blade bent, but the flywheel shear key will have failed.
Check out a site called Teryl fixes all, he is a bit of a character but knows mower repair very well.
Hopefully this mower did not bend the crankshaft end where the blade attaches, if it did, plan on getting a new mower.
Sorry, a shop will cost you a 1 hour "diagnostic charge" just to tell you it is beyond repair.