i'm not sure if this will help in your situation or not, but i had some problems similar and this is what helped me....
i assume you've also got an external keyboard and mouse. if you are using both the macbook monitor AND the external monitor at the same time, you don't have to have the power cord plugged in (at least in my experience), but if you're using ONLY the external monitor (not both at once) then the power cord MUST be connected. then put your computer to sleep, close your macbook, wait a few seconds, and then move your mouse and it should come up and be good to go.
if your problem isn't related to that, then i'm sorry to say i have no idea what to tell you.