Workstation to Server Connection

I am having a small issue…. I have currently set up an small office with 6 pc’s running XP Pro and a Server running small business server 2003. I have created a domain for them, which they are currently logging into. They were fine for about 2 days than suddenly they are complaining that the pc’s are loosing connection to the server thru out the day, which is causing the pcs to lock up. They are get a message that is trying to connect to the server and they eventually do. Why would the connection drop??