There was an update of the Steam software today which for some strange reason creates a new folder in C:\Program Files (x86)\Steam\userdata\0\ and then copies and deletes from \documents\my games\fallout4\ the .fos folders meaning that the saves will no longer load. It does the same for the Skyrim LE .ess files as well.   Although this is easily remediated by copying the files back into their respective folders in \documents\my games\ it is an unwelcome error on the part of Steam.