My mower had a similar problem. Turn mower on its side with petrol tank lower, and air filter uppermost (to avoid air filter being flooded with oil). Remove cutting blade (fixing bolt has normal l/h thread), then remove screws holding plastic cover, to reveal drive belt to rear wheels - may have broken or just jumped off pulleys at either end(replacement belt costs about £7). Refit belt, cover and blade.
Martin Lofthouse, Somerset, England, July 2008