Laptop Monitor won't wake up.

I leave my laptop plugged into an external monitor most the time and keep the laptop closed. When I open it again I can't get the monitor to display anything. It works fine when I boot up. I've experimented a bit with having the laptop monitor go to sleep or turn off, and it wakes up fine (after a few seconds). The external monitor always seems to work.

I'm using an HP Pavilion dv6000 (AMD processor)