Double check your monitor connections and make sure you have the monitor set to the correct input selection. Most monitors with multiple input connections will have a button to select the correct input and if the monitor is set to lets say VGA and you have the connection on DVI it will not show a single because there is nothing coming on the VGA connection. Other than that try it on a different monitor and see if it works on that.
One more thing is the PC turning on? The power light comes on fans spin and such? The reason I ask is because there may be a on off switch on the back of the Power Supply that might need to be turned on also. It would be a rocker switch one way on the other off.