My external monitor said "can not display in this mode"

Help!!

I hooked up my laptop to an external LCD "dell Ulstra Sharp 1700". It worked fine before.  But I disconnected my laptop from external LCD a few days ago, yesterday, when I reconnected it to the same LCD, it kept saying "can not display in this mode".  I unpluged and pluged, at one point, it worked.  I did not disconnected it after I turned off laptop yesterday.  This AM, I turned on my laptop, this time it came back the same message "can not display in this mode".  When I pushed the button in the LCD, it won't give me the manual like before.  Also, it showed "In power save mode. please enter any key".  But when I touched the keys in my laptop, it did not response.  And nothing I did can help the LCD comes to work.  Please help.  What is wrong with my LCD.