Well....
I don't know if this will work, but try it anyway.
Reopen the file again. If that same dialog comes up, choose VIEW, so that the one in memory stays.
In the future, I strongly recommend manually saving every ten minutes. LabVIEW does not, and probably will not, have an autosave feature. If they did, I would disable it. Saving your work should be a matter of discipline. I once lost four hours of work because I didn't save. I learned my lesson that day, let me tell you...
Good luck, and I hope the damage wasn't too bad.
As a rule of thumb, I save whenever I make any difficult changes, or on average, about every 5 to 10 minutes!