Vista doesnt recognize my external monitor sometimes?

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I connected my macbook to external monitor.
This morning I woke the macbook up, the external monitor wakes up. Since Im using wireless keyboard, I closed the lid. And the macbook went to sleep again.. Then I open the lid and wake the macbook up.. but this time, the external monitor didnt show anything..
I went to try to change the display to external monitor.. but the external monitor didnt show up..

Vista sucks...