Hey there, how you been mate? I'm assuming you've checked that the shift lever is still connected to the small linkage and that this is connected to gear actuator on the side of the engine. Pretty obvious really but I've both come unscrewed at various times.
Assuming your problem is more serious it does sound link the detent spring, which would mean you're also jumping out of gear and finding it difficult to engage pretty much any gear. The fix isn't as bad as you might think so don't let anyone tell you the engine has to come out. It can be got at from the front of engine. My sump also had to come off because a piece of the spring went missing but that's not always necessary.
I wouldn't try doing it myself but I'm sure someone like Warp will be able to "talk" you through it if you're that way inclined. Good luck.