One last thing i forgot to say. Some ethernet cables (from the workstations to a switch) are quite close from power cords.
Although most cables (including the one from the main switch to the server) are at a proper distance from power cords, could EMI be blocking the whole network?.
EMI can be a possible cause if all network activity is disrupted. Based on your initial post, you only encounter issues with file sharing so EMI cannot be the cause.