Hi,
Sorry to hear that. 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 :)

Thanks, this fixed everything :)