itch.io

Level it up while holding a Prism Scale, I think.

If you have gone through a Pokémon Center during that time the game makes an Autosave in the save folder.

In C:\Users\(Your name)\Saved Games\Pokemon_Uncensored

Delete (and backup just to be sure) your original same Game.rxdata and then rename Game_Autosave.rxdata to Game.rxdata.

Hopefully that restores the progress you made.