[Zope] zope username /password problem on our portal

we are running a web portal using Zope created by our previous administrator who doesn't work with us anymore. The portal is up and running. The problem: After logging in as one of the portal users when trying to access any information, it is now gives a pop-up as Enter Username and Password for Zope. The site has been working for about a year now without problems. I don't know anything on Zope but am a another system admin. Any help would be very appreciated. thanks, shyam