Why wont my lawn mower move?

I have a yardman riding mower with a briggs&stratton I/C engine. I was using it in 4th gear going up a hill when it stopped moving. I turned it off and now i cant get it to start. I tried both the keys start and the pull cord. I didnt change any tires lately and it ran fine yesterday. It has new battery, spark plug, and has always started. I need to get this working or my lawn will be 1 foot tall. when it wouldnt move I turned the mower part on and it shook a lot. what could be causing it not to move? Why wont it start? and how can i fix it?