Network goes up and down

One of our 16 sites cannot connect to the network. We even put in a new server 3 days ago. They can connect for 3 days, then the server goes down for them. This has been continuous. When you go to logoff, their name is there. They cannot access email or anything. What did we do wrong? We use a DNS for internet, WINS, Gateway. We use routers and those have been checked. This has been happening for 4 months. Before that, there were no problems