Sometimes the blades get gummed up with leaf juice - but you'd probably feel the motor buzzing or humming. If gummed up dismantle and clean otherwise...
If you hear the motor turning and the blades not move then there's a break in the linkage - belt - toothed cog - whatever.
To discover the actual fault it will require disassembly - there is no reset button.
Good luck...
M Don, October 2014
