Outlook Disconnected From Exchange 2003

I have a setup of 5 PC (Vista & XP) running Outlook 2003/2007 – connecting to SBS 2003. Setup exchange and it worked fine for 2 days, then all pc’s lost connection to exchange in the night. I can use OWA remotely ok but on the LAN OWA will only work if I replace the server name wit the Server IP address. ie) http://192.168.2.1/exchange. Could this be a DNS issue and if so where should I be checking.
Please can somebody help.