As far as I know, "s" for saving (like in the predecessors) is still not implemented (it has been reported several times, but I haven't seen a fix until now).
I didn't try this before, but perhaps you can find some temp file of your map. If you can access the Win Explorer, you could try to save it to another place and try to open it after a restart. (Not very likely, I have to admit. Probably you can't copy this file because it's open in MP. Probably you cannot load a temp file of a previous session in MP.

Don't know, but it's worth a try.)