It is usually the ratchet pawls are not flicking out and engaging with the crankshaft plate, either remove by taking the 3 nuts off the studs or try some WD40 sprayed into the mechanism through the vent and leave for a while. It may have got "sappy" grass in there last season and it has set "gummy". no other reason if it was working fine when you put it away.
Chester Mower Man, February 2014
