How are you trying to open it? Through Outlook or just double-clicking on it? What Office versions are on the computers?
Do the files still work fine on the old computer?
If the file works fine on the old PC but not on the new one, do a repair and compact of it in the working PC, then try to open it again on the new computer.
If you are trying to just double-click on it, open up Outlook and go to File, Open, Outlook Data file, see if that works.