Sounds as if the top plate will have to come off so you can see whether it's just the dial itself that's stuck, or the selecting mechanism that couples with it.
If you want to try it yourself, you can do it -- if you've ever removed a top plate before, this one is pretty normal. It's been a long time since I've done this, but only tricky bit I recall is the shutter speed/ASA dial -- make sure you note down the settings before removing it. And be careful when reassembling it -- it's easy to bend the ASA disc.