Lost 3rd gear
 

Hi, everybody! So i just bought a gsx f 600 bike without 3rd gear. What coud be the problem ? 1,2,4,5,6 - is ok. But when i put in 3rd gear..nothing happens..there is no weird sounds in engine/gearbox..the wheel just dont spin.
but it stays in 3rd gear..it doesnt pop out. thank you.