When you turned it on its side you allowed oil into the cylinder.
Blue or white smoke coming from your engine usually indicates burning oil, which can be caused by:
Overfilling the crankcase with oil
Incorrect oil grades
Operating engine at greater than a 15 degree angle
Inoperative crankcase breather
Crankcase air leak
Blown head gasket
Worn cylinder and/or rings
Turning/tilting the engine on its side for storage, oil change or any other reason.
Note: If the engine has to be tilted, ensure the spark plug is facing upwards to help prevent oil from escaping the crankcase.
Other reasons for blue or white smoke from engines include going over the engine’s oil capacity, an obstruction in the breather tube (located behind the air filter), damages to the cylinder/piston rings, and more.
Engine oil capacity is dependent on your specific engine model and can be determined by checking the Oil Recommendation FAQ. You can also learn how to check the oil level and change intervals to ensure the engine is running at the correct oil capacity.
How to Troubleshoot Engine Smoking
If you wish to troubleshoot and repair the small engine yourself,
Step 1: Are you tilting the engine or using it at more than a 15 degree angle? Tilting your mower or equipment can cause leaks. Right the machine before moving forward.
Step 2: Checking and changing the oil is an easy troubleshooting technique for this problem. Check to see if the oil is overflowing or you’re using the wrong grade/type.
Step 3: Check the crankcase for leaks.
Step 4: Is the head gasket blown?
Step 5: Are the rings and/or cylinder worn?