Lawnmower won't start after replacing parts?

I had to replace the oil seal on my lawnmower. This required me to take off the bolts on the oil pan (engine assembly) in order to remove the old oil seal. I ended up changing the spark plug, oil, and cleaning the air filter after that. Where could I have done something wrong because now the machine won't even start up after any amount of pulls. Could it be I flooded the carb, or something else? Any suggestions?