Busted or missing shift return spring. Some bikes it's a piece 'O cake to replace, and others... Either way, it means popping off a cover or two.

Example:
Got a service manual?

 

Download a service manual(if you don't already have one). You'll have to pull the clutch cover off, then the clutch and clutch basket to get access the the shifter ratcheting mechanism. Should be a pretty simple task, just take your time removing and then reinstalling the clutch, it's not complex, but real easy to misplace parts or get them in the wrong order