Mine has done the same. Look underneath, I expect the belt has come off.
Easy to fix, remove the blade, normal thread R/H, pull off the pully wheel and spacer piece. If you have trouble undoing the bolt, fix a small G clamp to the body to stop the blade turning.
Place belt on small pully at the rear. Place belt on main pully and then ease the unit back onto the spindle. Make sure the little "wedge" on the spindle is lined up with the groove in the pully unit.
Roger
July 2010