can't get out of closed-lid mode

I'm using an external display with my macbook. Following the procedure described by apple, I put my mackbook to sleep by closing the lid. Then I connect the display and power cable, and wake it up by hitting a key on my (wireless) keyboard. But then if i put it back to sleep, disconnect the external display and the powercord, and opening the lid, the computer does start up, but the display stays black. It must be thinking there's still an external display connected. The only way to deal with this is to restart the computer. Does anybody know an easier way?

(I've also tried 'detect displays' when lid is open, but nothing happens)

Thanks,
Ben

(MacBook 2.0 White, OSX 10.4.8)