I'm sorry I don't know anything about growing grape vines, but I wanted to let you know I read your post and know the feeling when something goes wrong and desperately want advice.
My vague at most understanding is that grape vines could take quite a bit of pruning, and it is in fact desirable, so even if you won't get harvest from the pruned portion and you might not save it, the main plant/vine should be OK and would grow new shoots.
Could you post photos or at least a rough sketch of the vine and where you cut it off? Let's hope more experienced members will come along and provide a more definitive opinion.
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