XenServer 6.2: NICs showing as disconnected despite being connected

  Sometimes if you connect a network cable to a NIC XenServer doesn’t register it and keeps the status as disconnected; if this happens, try the following via SSH (or from the console, though you will be kicked out of XenCenter once you do it):  
xe-toolstack-restart
 

It may take a little while for the restart to finish but once it does the NIC should be showing the correct status.