External Display connected to my MBP problem

I hve a display connected to my MBP running Snow Leopard, and i have no issue when the MBP is open, and both screens mirrior just fine, my problem is I want to close the cover of the MBP and just view the external display, but when I try it puts the MBP to sleep, and nothing I do can wake it up, short of restarting the MBP. I have zaped the pram, shut down the MBP, disconncted the power for 15 sec, restarted.

What else can i do?