Your Microsoft Office file goes missing in Dropbox and can not be located.
Temporary file in Dropbox has replaced the original file.
It is normal for Microsoft Office to open a file such as myfile.doc and while it is being edited make a copy and name it ~my file.tmp.
When the file is saved the original is deleted and the temporary file is renamed back to the original.
When some applications (such as Microsoft Word, Money Excel, or PowerPoint) open a file, they will often save a temporary file in the same directory and name it in one of the following ways:- Name begins with ~$ (a tilde and dollar sign) or .~ (a period and tilde) Name begins with a tilde and ends in .tmp, such as ~myfile.tmp http://support.microsoft.com/kb/211632/en-us/ ("Description of how Word creates temporary files").
Dropbox does not sync these temporary files on any operating system.
Every time this has happened in Dropbox ,I have recovered the file by renaming it and the extension ( tmp) back to the original file name.
This is still a mystery but I think that the Office application must be open when switching off the PC and has not completed the action of renaming the .tmp file but has deleted the original.
I am still investigating.