Remove the crank pulley bolt and the pulley itself and inspect both for damage. Either you had the very common pulley bolt breakage or the keyway on your crank pulley wore out, but most likely it is a combination of both. Count your lucky stars if the bolt hasn't broken off in the crank.

The key should come out easily when the crank sprocket is removed. To get this far, you're in for a lot of labor - it's the equivalent of a timing belt job.