Remove the spark plug and see if it has oil on it. Tipping the unit may have let oil into the cylinder and the plug is fouled. Replace the spark plug. If unit is overfull on oil, drain it down to the full mark, if overfull, it will suck it into the air cleaner and then into the cylinder. I would try that first, and then if it still does not work, remove the carburetor float bowl and check the float and needle valve, tipping the unit may have made the float stick, so now is not moving up and down, not allowing fuel into the cylinder. Here is link to Briggs carburetor, yours will be similar.http://outdoorpowerinfo.com/repairs/briggs_walbro_lmt_carb.asp
Sorry for delay in replying to you.
You've put me on the right track to fixing the mower, and to learning something about maintenance. (As you can tell, I'm not very practical but I'm going to change that).
I have to get the right tool for removing the spark plug, and will be sucking some of the oil out and replacing the air filter. Hopefully that will work but I haven't fixed it yet, as we're having bad weather here at the moment.
All the best,