There is a computer in the morning suddenly unable to get online, or good Friday, suspected of being blocked (access control lists or port is closed <principle of proximity>).
IP address corresponding to the MAC address found in the computer, enter ipconfig / all obtained under cmd.
Into the switch, execute sh mac-address-table | last four digits in MAC addresses (note lowercase).
Not shown, the description is not connected, the port may not open.
Find the corresponding IP VLAN number.
Redisplay VLAN state, sh vlan, find the corresponding VLAN interface list.
Display specific interface state, sh int interface name (gi3 / 12, etc.).
If it is off, the process proceeds int gi3 / 12.
Open the no shutdown.
A little later on, do ping IP address or performing sh mac-address-table | after four in MAC address.
If the former is closed, not open the port, shutdown on the line.