If your keyboard suddenly stops working, the first thing to do (before sending a helpdesk ticket) is to check the cable. (Excluding laptop keyboards) Make sure the keyboard is actually plugged in.  This may seem simple, but it's often overlooked.  When you trace the cable you may find a USB end or a PS2 connector going into the tower.

 PS2 Connector for a Keyboard

If the cable is unconnected, plug it back int to the appropriate slot.  A USB connector can go in any USB slot while a PS2 slot should be round and color coated purple to match. Only a USB type of keyboard will hook up quickly.  (If the keyboard doesn't work immediate check for any indication of drivers loading on the task bar:  picture below) After any drivers are loaded (usually 1-2 mins) the USB keyboard will be up and running.

THE PS2 CONNECTOR WILL NOT RECONNECT WITHOUT RESTARTING.  PS2 connectors are an older style of connector.  It will need to be connected when the computer first boots up after being completely off or restarted. A computer with a connected PS2 that hasn't be restarted is a common issue.

If the cables are connected from the start of the issue, unplugging and reconnecting may resolve the issue.  SEE ABOVE TO ACHIEVE THIS SUCCESSFULLY 

If the keyboard is still nor working, is working incorrectly, or you need help with the above steps, send a helpdesk ticket.