I've just had a similar experience and I'm wishing I had read this thread before re-saving.
I was working on a design, let's call it "Basin", up to version 29.
I clicked "Save", then after about 10 seconds I closed Fusion. It gave the usual message that it would close once files had finished uploading.
I came back half an hour later and re-opened Fusion. It gave a message saying that a recovery file was available, so I opened it (I guess the network connection had been lost during saving). The window title was then something like "Basin (recovery file) *". [The old v.29 file was from about an hour earlier and I had made a lot of changes inthat time].
I didn't want the file name to be "Basin (recovery file)" so instead of "Save" I clicked on "Save as" and selected "Basin", thinking it would save it as a new version (v.30) of the original design.
Instead, it created a new file with the same name, starting at v.0.
My problem is that Basin v.29 was inserted as a component in a larger assembly. I cannot switch that to Basin v.0 because it sees the new file as a separate design. I have tried looking for the recovery file under AppData\...\CrashRecovery but it seems to have been deleted (and it's not in the Recycle Bin either, and "Open recovered documents" has no entries).
I could insert Basin v.0 as a new component in the assembly, and get rid of the v.29 component, but I think if I do that any drawings of the assembly will get very confused about which objects are visible, dimension relations etc. Fortunately I haven't finished the drawings yet, but in principle this brief moment of carelessness could have cost me many hours' work.
It seems a bit of a fragile system. Is there any way of getting the recovery file back so I can try again? Or redefining v.0 as v.30 of the old file?