I'm connecting my MacBook Pro to an external monitor. It works initially, but a few seconds after I close the MacBook the screen goes dark. Also the laptop battery isn't being charged by the cable from the monitor any longer.

I'm connecting my MacBook Pro to an external monitor. It works initially, but a few seconds after I close the MacBook the screen goes dark. Also the laptop battery isn't being charged by the cable from the monitor any longer. Why is this happening and how can I get the battery to charge and the computer to keep running on the external monitor? Thanks