This once happened to me on my old Nintendo⁶⁴. My little sister accidentally (or purposely) deleted ALL of the saved data in Dr. Mario⁶⁴, so all of mine, my mom's, and her siblings' high-scores in the game have all been wiped (I think the highest of them all was somewhere in the 70-700k points I think...).
I then had to play the game again and try to remember what the high-score was and then try to beat it (I was completely capable to do it, but I've never had enough time to do it since it would go on for >2 hours). Then I had to recomplete the story mode, (which wasn't great) and so on...