If you have an encrypted file, and the user who encrypted it has left the company taking the password with him, then the file is useless (assuming it was encrypted with a reasonable algorithm rather than something home-brewed) unless you can contact him and ask for the password.
If you need the user ID to decrypt it, then recreate the user and see what happens.
But my guess is that you are looking at a useless file - encryption algorithms are very good, and it is extremely unlikely that you could "brute force" decrypt the file in this lifetime, unless processing power ceases to follow
Moores Law[
^] and fundamental changes to the design of computers occur.