I went into my SmartThings app today and found that all my devices have gone back to how they were in December. Some switches have gone back to their temporary names I use such as Christmas Tree lights and newer devices are showing as offline in the new app and are missing in the old app. It’s as if it all got restored from an old backup, but I haven’t done anything like that.
Individual customers do not have backup capability on the SmartThings platform at this time. However, your account in the cloud can get reset if they are doing some kind of platform maintenance. It doesn’t happen very often, but it has happened. so I would start by contacting support and let them take a look at it.