I have a Honda HRX217 push mower. Backfired when trying to start it, now it won't start?

It was running, then I shut it off to open and close a gate, and when I went to start it again there was a really loud backfire and now it won't start. It will sound like it's almost about to start but not. Then get progressively weaker each pull. I changed the spark plug and cleaned the air filter. Any ideas?

The flywheel key is either bent or broken. It is a very simple fix. Just replace the key and it will start. This video will walk you through the repair.