Hi zerojun! Thanks for the feedback. The game saves your progress in a local file within your browser. If your browser is set to automatically clear cookies, history, or all website data when you close it, your save file will unfortunately be removed.
To fix this, you can either add gamersz.itch.io/zenith-miner-v6 to your browser's 'exceptions' list for auto-clearing, or – much simpler – turn off the 'Clear cookies when closing the browser' setting in your browser's 'Privacy & Security' settings.
Hope you have fun and please let me know if everything is working now.