Could be one of two things:

Worst case, the slope cause old starvation and the engine began to seize due to lack of lubrication. Bad news, because engine is now probably shot.

Better option (and maybe more likely). Slope cause oil to run from crankcase breather into carburetor and you were burning oil. Muffler is now full of oil and will continue to smoke. May actually be less smoke at start up and then really begin to smoke as engine warms up.

Suggestion for either case: Check oil level. Make sure you are above "ADD" but do not increase all the way to "FULL". Check air filter element and see if it is saturated with oil. Replace if necessary (you can't clean a paper element); must be replaced.

Now start mower on level ground and continue to run at operating speed. If engine seizes after a few minutes, you had a worse case scenario and you need at least a new engine, and probably a new mower. If it clears up and quits smoking, and continues to run strong, you are OK and didn't do any long term damage.

'Better option (and maybe more likely). Slope cause oil to run from crankcase breather into carburetor and you were burning oil. Muffler is now full of oil and will continue to smoke. May actually be less smoke at start up and then really begin to smoke as engine warms up.'

This is what happened.

'Suggestion for either case: Check oil level. Make sure you are above "ADD" but do not increase all the way to "FULL". Check air filter element and see if it is saturated with oil. Replace if necessary (you can't clean a paper element); must be replaced.'

Do this.