I assume this is on the web version? Unfortunately, each type of browser has different rules for how they manage local data, but if it’s gone, it’s probable gone for good. There is a “save to disk” feature that can allow you to create a portable save file that exists outside of browser storage, but that doesn’t help you get your old save file back.
Since I can’t help you get your file back, here’s some advice for the future. Safari in specific purges local storage roughly every seven days for some unknowable reason, so if you’re using that browser, it’s likely to delete your saved game regularly. On mobile browsers there are strict limits to the amount of space they will allocate to local storage, so they are also unreliable, and you’ll need to save to disk frequently. For most other desktop browsers, saves should persist for months or years, if not indefinitely, as long as you don’t delete them yourself. Various software utilities (anything that deletes “temporary files” or “cleans diskspace”) will likely delete your browser’s data, and of course if you delete your own browsing data in your browser settings, that will also clear the data, so be careful with that sort of thing. Still, it’s a good idea to save to disk if you aren’t planning to play the game for a while.
Sorry I can’t help more :(
Note: The server doesn’t really have that much info, so you can’t recover your save via your multiplayer ID. Only your servant’s info is tracked, and records for players that don’t log-in for a while are eventually removed. Of course, in normal circumstances, your record is re-added next time you log-in.