Unfortunately I can only guess here, but it might be worthwhile to look at
~/Library/Saved\ Application\ State/com.apple.Safari.savedState/ (that's a symlink to
~/Library/Containers/com.apple.Safari/Data/Library/Saved\ Application\ State/com.apple.Safari.savedState/). It seems the information is saved as raw data (except one general plist file, maybe that can at least give you some hints), but perhaps you can simply replace that from a TimeMachine backup. Be careful with that however, perhaps the reason why the tabs are gone in the first place was a change in the data format.
If that does not work you can try to look through your history (⌘-Y in Safari), though that's gonna be tedious.