Back wheel not turning when throttle is applied

Hi ,

I'm hoping someone can help me diagnose a problem with my Honda SFX 50cc moped.

All was fine until the other day I started it up and noticed the rear wheel was not spinning when the throttle is turned. I've already checked the back wheel splines and the shaft splines and they are fine no wear whatsoever, I've had a look inside the crankcase as well all cogs are fine belt seemed tensioned, the only thing I notice without taking everything off is the compression coil seems very stiff as if its locked up.

Does anyone know whats the matter please, any information will be much appreciated!

Thank You, Jason