crank output cog

[kinkyfreestyler]

went for the mot today lovely weather , bike was running smooth and fast , never made it half way I lost drive, gearbox was fine, chain still in place clutch seemed to be working.
So long push home, strip the clutch out and the cog on the end of the crank that drives the clutch was floating around freely .
Never seen this before ! what am I supposed to do ? drive it back on or is it a strip out of the crank to have it reset on or crank rebuilt ?
any advice please .

thanks