How to fix my mower?

I'm using my self propelled mower and it stopped propelling itself. Then, I could push to move it, but if i tried to pull it backwards, the rear wheels would lock up. Any idea what's wrong?

Answers:
Tough without knowing the model, etc. It's definitely in the drive mechanism, but there are soooo many types. .. If you're not the type to tear it apart yourself, take it to a repair shop.

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