Could be many things but one of the most common is a blocked or oiled up air filter (usually because someone has tipped the mower up to clean the underside and sump oil leaks out of the carburetter intake into the filter)
Unscrew the cover to, or remove the foam rubber air filter and soak it in paraffin to flush out the 'gunge'
dry it in a paper towel and refix.
Geoff in Wales, July 2011
