Missing connectivity

Hello,
i have a small network in my shop there i have a server and from that five more systems connected. in the main server i had my accounting software.for the past few days i cant able to connect or operate the account software there is indicating network cable unplug but it is connected to the HUB (Switch).i had change the jacks and check everything is OK. but still i cant able to connect.please give me the solution for this as i am facing a lot with this situation.looking for a best an early reply.