I don't have any personal experience of the camera but might be able to give some pointers.
Can you cock and fire the camera? If not, first thing to try is to manually release the wind lock and recock the camera and fire it. This often sorts out problems where the shutter/mirror/wind mechanisms have got out of sync. The wind lock is most likely under the base plate and will consist of a lever engaged with the bottom of the wind shaft in a way that prevents the wind-on from operating.
If you can cock and fire the camera, then it sounds like the mirror mechanism itself and that would probably mean pulling the mirror cage to effect a repair. Not something I'd recommend.
Probably not of help but here's a picture of an XG-? where the coupling has come adrift under the base plate (arrowed). This lever which is normally engaged with the bottom of the wind shaft will (I think) cock the shutter. Could be something similar has happened on yours and it would certainly be worth looking under the base plate and seeing if anything like this has come adrift.
If you're really serious, you could buy a service manual from here.
http://www.lensinc.net/minoltarepair.htmlYou could also ask the question over at kyphoto
http://www.kyphoto.com/classics/forum/messages/2/2.html?1278744299. I did search the archives but couldn't come up with anything but someone may know the answer.