When i launch a game the monitor goes black and says "No Signal" then restarts

[Tordero9]

When i launch a game the monitor goes black and says "No Signal" then restarts.
Plzz Help!1
it didnt happend b4

 

[sosofm]

What GPU and PSU you have ?

 

[yoji]

and are you trying to pump a resolution.. or refresh rate to the screen that it does not support?
Check it out..

 

[Tordero9]

What GPU and PSU you have ?

NVIDIA GeForce GT 640