Internet disconnected approximately two hours later.

Windows 2000 server is installed in my server.There are 10-12 clients are connected to the server.ISDN line is connected to NT-1 box, and NT-1 is connected to HUAWEI router BRI card. Problem is that if we don,t surf net on server 2-3 hours later server became disconnected from the internet. while isdn line and NT-1 is working ok .