Migrated a site recently, and got this error when I was bringing it back online in its new environment:
Call to undefined function: field_file_urlencode_path()
What fixed this for me was to go to /admin/build/modules and make sure FileField was enabled.
When I first loaded the module page, it was unchecked - and actually greyed out so I couldn't select it, but after scrolling to the bottom - hitting save, it mysteriously re-enabled itself and everything was magically fixed!