Hello,
Sorry to hear about it. There's a backup file in the save folder.
You will find the save folder, on Windows:
On Linux:

In the folder you should find save_data.dat and save_data_backup.dat

• Make a copy of those two files in a different folder, for safety.
  
• Disable the Steam Cloud Sync for Chonkers (Right click in the library > Properties)
  
• In the save folder, delete save_data.dat and rename save_data_backup.dat to save_data.dat
  
• Start Chonkers and if everything is restored close the game and enable the Steam Cloud again.
  
• Next time you run Chonkers the Cloud should give an error. Simply confirm the use of the most recent save file.

Please, let me know if you need more assistance :)

Originally posted by Rauchen:

Hello,
Sorry to hear about it. There's a backup file in the save folder.
You will find the save folder, on Windows:

%appdata%/godot/app_userdata/chonkers

On Linux:

.local/share/godot/app_userdata/chonkers

In the folder you should find save_data.dat and save_data_backup.dat

• Make a copy of those two files in a different folder, for safety.
  
• Disable the Steam Cloud Sync for Chonkers (Right click in the library > Properties)
  
• In the save folder, delete save_data.dat and rename save_data_backup.dat to save_data.dat
  
• Start Chonkers and if everything is restored close the game and enable the Steam Cloud again.
  
• Next time you run Chonkers the Cloud should give an error. Simply confirm the use of the most recent save file.

Please, let me know if you need more assistance :)

Thank you so much for your fast and helpful response. I tried your suggestion and it's worked perfectly. Unfortunately, my backup was from 3 days ago so I still lost a bit of progress, but I'll absolutely take it over starting afresh. Much appreciated and much respect, keep up the good work :)

Do anyone know why this happen ?