Check if monitor is dead....check that u r graphic card is seated well and extra powered(if needed)...and of all...check if ur mobo is gettin power and fans are runnin
Elaborate your problem. Monior not turning on or cpu not working?
check if your monitor is working on any other PC, check if any other monitor is working with your pc. the problem will be isolated.
tell us if you have a separate graphics card or you use onboard one.
try to be as detailed as possible.