[OsoMoore]

Take off the inspection port and look inside while having someone rotate the stick side to side. You should be able to see the selector rod move the fork selection pin between the different forks.

If you can't go into the 5/R plane, then you'll probably see that one of those gears is engaged. In other words, you'll see that the rod holding the 5/R fork is moved one way or the other but the selection pin isn't in the slot on that rod. That will show you which gear it is stuck in. You may even be able to get it out of that gear using a screw driver through the inspection port.

Did you get to see the inside of it when it was open? Once you've seen it apart in front of you, what you see through the port will make a lot more sense.