Unfortunately, if you didn't have any backups, that data is probably gone for good. File recovery software probably won't help to restore iPhone bookmarks
, though it can't hurt to try. It's possible that Safari created a new Bookmarks.plist file in a temporary folder, wrote the changes to it, then deleted the old one and replaced it with the new one. (I'm not sure whether it would use that technique internally, or would just open the file, write the changes to it and then close it again.) If it does work that way, the old one may still be present and not overwritten. I wouldn't say that's likely, though. Try it with iPhone Data Recovery
and see what you get.
This should be your cue to start backing up ASAP, before you lose something more important!