VPN Problem

I have the Remote Access Service (VPN) set up on an NT Server in our home office. Users dial in to access email and to run applications remotely. This has worked fine for over a year. Suddenly, they are getting error 619: The port is unavailable, when trying to connect. The laptops they are using are Windows 2000. I have tried reinstalling all services on both ends with no success. I have not installed any new services or programs on either end. Any help would be appreciated.