If you tipped your mower over with the carburetor side down, oil could very well have got into your carburetor and air filter.
Remove your air filter and look for oil soaked into the element. Check for oil in the air filter housing. If it is oil soaked or very dirty then you will need to replace the air filter. If oil appears in the housing then clean it out with a clean rag.
Remove your spark plug and check the colour. Is the plug oil soaked? Is the plug black? If the plug appears black or carboned up you had better replace it. If it is oil soaked then wipe off the oil as best you can and carefully clean in and around the electrode. A quick spray of carb cleaner will help dissolve the oil. Make sure you don't alter the gap between the spark plug electrodes as you clean.
Check your oil level. Is it over full? Is it low? Can you pull the rewind cord or is it really hard to pull it over? If the oil level is over full you will need to drain out some oil to correct the level. If the level is low then add some oil to correct. If your pull cord was hard to pull and your oil level was over full, is it easier to pull once the level has been corrected?
Once you have gone through these checks, try to start the mower again. Will it start? If not, remove the air filter again and put about a teaspoon of gas down the hole into the carburetor throat. Now try to start it again. Does it start?
If the mower starts, you will find that it will smoke for a little while until all the excess oil has burned out of the cylinder and the muffler. After that it should clear itself. If the mower still wont start your carburetor may need cleaning.
Please let me know what happens. If all is well and your mower starts and you are satisfied then please click on the Accept button. If you still can't start the mower please let me know and we can then go further into a solution.