If the second cutain didn't close, it is probably just a felt light seal come unstuck, blocking the second curtain.
Open the back, and look closely the the top and bottom seals and leading edge of the second curtain. It should be obvious which seal has come unstuck.
The locked up gear train is normal. It is in the middle of a very long exposure
Don't try to force anything. You can use a jewellers screw driver to carefully ease the seal back under the shutter curtain, so the shutter can close. It will just jam again on the next shot though, so you will need to fix it.
This happened on my M2 in spring. Each company I asked for a quote offered a CLA. This is understandable, as here in Europe they have to offer a 2 year warrantly, but the camera only cost me €250 ten years ago, so I made up the special tools, and fixed it myself. Turns out my camera is in much better condition internally than externally. It has had an expensive CLA at some time which obviously involved replacing a lot of worn out parts, as I could find no evidence of wear ( the outer shell tells a different story).
You have to strip the camera down a bit to remove the light shield, upon which the light seals are glued.
Here is a link to my write up.
This is not a differcult repair. You can buy the special tools over ebay.
search for "leica tools"
This is a good way to take away the fear. It is much more robust inside than you think.