alex, since I am not sure what mower you have, here is a somewhat generic answer. First make sure that your belt is routed properly and that your blades can spin freely. You check that with the engine OFF (of course, but just to make sure ;-) If the mower runs normal with the blades disengaged, we could for now rule out poor compression, bad timing, bad air/fuel mix etc. The thing to check are the safety switches. in general mowers will have at least two. One on the seat to make sure there's an operator in it, and one on the transmission, clutch or clutch linkage somewhere to prevent mowing in reverse. I'd check the seat switch first. Let us know if this helps. Good luck.

I have a Yard King mower with a Murrey engine. It did this once before and it was a bolt that had come off and we just had to reconnect it. But I can't remember where it was.