Hi Ottoknowbetter,
I am sorry that your save has been effected by this error. What you experienced is called an MD5 error. This error occurs when the save struggles to load - most commonly after a crash - and the area is reset to it's day 1 state. Can you confirm if this occurred after a crash for you and what console you are on?
Unfortunately it is not possible to restore the save - Unless you were on PS4 and backed your save up to the cloud or externally - all saves are maintained on the player's console, we do not have access to them or a roll back ability.
All information on this error can be found here:
https://7daystodie.com/forums/showthread.php?50175-Known-Issue-Terrain-Rewriting-(MD5-Issue)
If you are on XB1 and find your game crashes regularly, there is a workaround other players found that can help avoid MD5 errors if done immediately after a crash (as this is unofficial and player found, caution is advised) -
https://7daystodie.com/forums/showthread.php?69218-Possible-work-around-and-or-solution-to-md5-crashes(base-wipes-resets)
Players also noted that active crafting stations at the time of a crash or a large crafting-storage area are the most commonly hit by MD5s at the moment. In future, I would recommend spreading items out if possible as it may help avoid MD5s also.
Both myself and the team know how frustrating and disheartening this error is for players and while it may not be of great comfort now, the team is working to fix the remaining triggers of this error, having fixed a large number already, and have assured that they are not giving up.
Again I am sorry you were hit with the MD5 error, if you have any questions I will do my best to answer them.
Clare