Computer cant close

hello

i have a computer with a p43d1600twins-1394 motherboard and when i in vista go to start---shut down it starts shutting down and after some time about ½ minutes the lcd screen gets dark and says no signal but i can hear that the coumputers fans stil are running and there are still light in my computer

if i hold the power buttum down a while the computer shut total down and the fans and light stop running put how can i fix that i just in vista can press shut down and leave my computer and when i am back it is totaly stut down