PC trun on/off problems

Hi,

I just installed new components into my PC today:
AMD Phenom II 720 BE
Freezer 64 Pro
Gigabyte MA770-UD3
Kinston HyperX 4096MB 1066MHz CL5
Powercolor 4870 1GB
Samsung S223L
Seasonic SS500ET

But the PC won't start. When i press power button, the PSU fan, graphics card fan, and CPU fan all switch on and work. The hard drive and the optical drive seems to work also. However, the monitor remains black and it doesn't find any signal from graphics card, and there's no sound prom the PC speaker after POST. Also, the PC cannot be turned off by holding down the power button, I must switch off the PSU button on the back side. The reset button does not seem to work either. Any ideas what the problem could be?