The first thing is to check whether you've accessed the journal article through the Library's system.
If you've done that, and you still can't access the article, there could be a number of possible explanations, for example, it may be an issue with the Library's access on the publisher's end, which is easily fixed. In the first instance, get in touch with your Faculty Liaison Librarian, and they can follow this up for you. Click here for the contact details of the Faculty Liasion Librarians.
If it's something that isn't available via the Library's website, you may be eligible for Document Delivery, which is a service the library provides to suport staff and students invovled in research and teaching by:
See here for more information on Document Delivery.