I think that your syntax tars an already tarred file "file.tgz" and the name of the created tarfile is "file". So the original contents of file were overwritten. However if the file.tgz didn't exist before the command and you received and error, you might still give "file" a look with the 'file' command. (To may files named 'file' here!)
/bin/file file
The 'file' command examines the file for 'magic bytes' to analyze what the file is.
If you haven't rebooted the computer, you could also use the 'lsof' command.
lsof | grep 'file'
or maybe
lsof | grep 'tar'
I doubt if a process still has the original file open, or even that the original file isn't overwritten, but it is worth a shot.
Sometimes there will still be a handle open that you can access by identifying the process that has the file handle reference and access the file through the /proc/<pid>/ directory.
