Failed to load map! Umap appears to be an asset file

Unfortunately, if the file is corrupted, and you are not using Source Control, the only way to restore the file would be to use a backup, which you have already stated is also not working for you. The best solution here would be to remove the maps and recreate them from scratch at this point. I recommend looking into using Source Control such as Git or Perforce in the future to prevent issues like this from occurring in the future.

Let me know if you have any further questions.

Have a great day