Self propelled lawn mower not self propelling?

Cut the grass this morning before the impending rain. All going well at first, stopped the mower to empty the first lot of cuttings. Re started it pulled handle back and it would not move :confused: , cable is pulling, mower is running ok. Had to finish off pushing the thing, disaster . Before I start taking apart, anybody got any ideas, it�s a Champion 375.

 

Sounds like it just went on strike!!

 

I know the feeling

 

Sounds like it's come off its cogs......

If you have time - this is a useful book.....

http://www.amazon.co.uk/s/ref=nb_ss_b/026-4944143-1193256?url=search-alias%3Dstripbooks&field-keywords=haynes+lawnmower+manual&Go.x=0&Go.y=0&Go=Go

 

Superb, many thanks