Bikes front sprocket won’t move but goes into all gears and gear selector shaft still spins

[Curtis]

Hi I have a Honda mtx125 that I bought not that long ago the bike is mint and goes very well, I changed gear into 2nd and screamed like it was in neutral and came to a stop the shaft that the front sprocket connects onto spins in all gears but front sprocket doesn’t spin with it, any help would be great cheers 

[Mississippi Bullfrog]

Sounds like the sprocket has come loose or the splines on the shaft have stripped. A photo might help. I'm guessing you've checked the tightness of the retainer?