It has gone nowhere, its in the current directory. It has been renamed as
.... (four dots).
As any filename having a
. in front is treated a hidden file, so it has become hidden. So, if you do
ls, you won't find it. Like always, you need the
-A) option of
ls to view the hidden files i.e.
ls -a (or
ls -A) to see it.
Let me break it down, you ran
mv filename ..\.. , the first two dots would mean the parent directory if it were
../, but you have used backward slash
..\ which indicates shell to escape the next character but a dot
. has no special meaning to shell. So, it will treat it as a literal
. and the last
. also added, so you got four dots
.... as a filename.
To revert back to the previous (original) name, run
mv .... filename.