My John Deere walk-behind mower won't start or even turn over after I replaced the drive belt and blade today. It's a 7-year-old JS63C with a 6.5-hp Briggs & Stratton 121602 Intek engine.

The belt broke 3 days ago when I ran over a small piece of cedar bark mulch in the lawn. Replacing the belt was no big deal and, for good measure, I decided to install a new blade as well.

With machine on its side as I removed and replaced the blade, the shaft turn slightly (just a few degrees) once or twice. And when this happened I heard a "belching" type of noise back through the carburetor/intake?

The mower has gas and oil and I've reconnected the spark plug, but the starter rope will barely move. Seems almost as though there's some sort of mechanical block.

Is this possibly a vapor lock? And if so what can I do to fix it?

Hi,
Probably the most confused term in existence; vapor lock is caused by the carburetor getting so hot that the fuel vaporizes and the pressure prevents the entrance of cooler, liquid fuel.
Your engine does not run. Did you slant it with the spark plug down? I am betting you did. Pull the plugand pull the engine over. Betting you get a spray of oil out the spark plug hole. Check your air filter , if so, as you have probably soaked and ruined it.
Always point the plug or air filter toward the sky (depending on how engine is mounted on deck.
Peace,
Clarke
PS I use a JS63 with a Diamond Plus from another machine.