Sounds like you may be running on 1 cylinder. Check for plug failure loose wiring somewhere. Start the mower, in park remove plug 1 wire, if engine runs, replace and remove plug wire 2, if engine dies, plug 2 is bad or cylinder needs troubleshooting.
I've had this happen before on a Kawasaki engine, The rod actually fell out of the guide and was laying under the valve cover, I replaced, adjusted, and it ran fine after that.
If not an engine issue I might look to make sure my spindles aren't locked up or in need of new bearings. This could cause the mower to bog down, but I'd think the belt would burn up after running it like this too long.
All that comes to mind.