If you are reading this, DO NOT RETRY TO OPEN YOUR GAME! Read the FULL post first.
While I was writing this question, I thought that
CCLocalLevels.dat might be the local levels. So, I copied all the
CC*.dat files and pasted them in a different folder. Then I deleted all the files in the GeometryDash AppData folder and restarted the game. The
CCLocalLevels.dat file changed from 30MB to 1KB and that was the most stressful moment of my last 5 years. I stopped for a bit to think clearly:
If there is both a
CCLocalLevels.dat and a
CCLocalLevels2.dat, then the second one might be some kind of backup. Now, what I think that happens is that the second file gets overwritten every time you save, create or delete a level. Later, I started to search for how to fix the broken files, as the only backup I had at that moment was
CCLocalLevels2.dat, but it still didn't work properly, provided that the game deleted the file on its own.
After some research, I came across this cool little software that fixes your GD files (link). I instantly understood what it did so I copied the
CCGameManager.dat, and the
CCGameManager2.dat files and pasted them in the
%localappdata%\GeometryDash folder. Make sure not to cut them, that's how I lost my first set of backup files. The program then told me that it successfully repaired all the files, but it didn't find the
CCLocalLevels.dat file (the lost one).
Now, if you try to download the software and paste the broken files in there (make sure to keep backup), the game should start and your levels will still be there. If you don't want this to happen ever again, just make sure to back up your files on the cloud (or locally) and close the game by its own X button (in the main menu). What happened to me was to close the game using
Alt+F4 or just turn off my laptop without closing the game first. All this problem started from my laptop not having a battery (it's broken. I only have its charger).