This tutorial will explain if you install any other file manager like pcman (This is your default file manager) etc and you want to change back to ubuntu default file manager (Nautilus) follow this procedure
(1) System--->Preferences--->System Settings
(2) Select Default Applications -> File Manager and select ‘Other' at bottom, then the ‘browse' option -- "..."
(3) Select ‘Add' from the ‘Applications Preference Order'
(4) Type in "Nautilus" rather than browse for it, click OK.
(5) Ubuntu associates this with ‘File Manager', adding this as an option and Select ‘File Manager'.