The save files are stored in ~/.local/share/godot/app_userdata/Citadel of Celestial Cascades/
I'm not sure why would it not start up, you could try redownloading the game and removing both files in the directory above. If you want to restore your progress run the game once so the save files are recreated, close the game and change the number in the first line in save.sav to 17.