What does your network setup look like? Has it recently changed (i.e. you installed a new piece of hardware for it, either to add to an existing network, or replace a failed part)? The reason I ask is because most routers don't permit file-sharing traffic to pass through them, even if this traffic is limited to the local side only. This is a common cause of file sharing connection failures - the solution is to use a hub or switch in addition to the router.