Seasonal autosaves are something else you might want to turn on (and I make mine do an on-embark, too, but probably wouldn't be necessary for you). I do this even though I've not had your problem. Just being OCD.
Anyway, perhaps with the above set, you would find that the Summer-In-Year-Six save opened, even if the subsequent Autumn and further seasons had not. But then you've only lost up to a couple of years of game-time play.
Remember that if you load the (I forget
exactly how it goes, but something like) "region1-Aut-1073" save and continue with that, then the next autosave will be "region1-Aut-1073-Win-1073" (and the one after that "region1-Aut-1073-Spr 1074") so once you've verified that the game you load is the one you want, rename it
back to plain "region1" (if you don't fancy changing it to "FortressName"), after deleting or renaming the existing, corrupt, version. Otherwise, the file/directory names could get messy.
(Wouldn't hurt the gameplay, at least not until after several such incidents broke the 255-or-whatever character limit, although it might also look messy on the Loading screen...)
(Oh, and several years of seasonal saves
might look messy in its own right, in that self-same loading screen. I also periodically (after exiting the game) go into the loading screen and create a "_region1" directory to shuffle any seasonal saves of more than year old into, to leave just the regular (undated) save for each embark, and up to four historic ones. I could shuffle them
all away, because one can always retrieve from the _region1 folder any "region1-foo-###" folder that I feel the need to test, old or new. But the point is that moving them another folder-level down means they don't show on the loading screen. I could just as easily delete them, but I'm OCD about not doing so, and
you're better of not doing so for reasons of making
sure you have an uncorrupted save you can get back to, even if it's a couple of game-years old.)
Sorry, does any of that make sense? A simple post expanded, immensely, during writing. Sorry.