Are you on an iPad, by any chance? If not, which browser and OS?
If you're on iOS: the game is saved into "HTML Local Storage" in the browser, and unfortunately Apple changed the default settings on iOS to prevent saving data to localStorage when hosted cross-domain (as is done here on itch.io). This was ostensibly done for privacy reasons, but unfortunately it completely broke saves for browser-based games on itch.io for iOS. The only work-around for SIC-1 (short of disabling the "Prevernt cross-site tracking" setting--which I don't recommend doing, since it's a global setting) is to manually export your save data (Options -> Manage Save Data) and then import it next time.
If you're not on iOS: let me know the browser+OS and whether or not any settings (especially privacy/security settings) have been modified, and I'll give it a try.