vga

hi all

my AOPEN P4 machine is not showing anything on screen, but its switching on, the fan is spinning proper. It doesnt have got onboard vga. I tried to change different kinds of vga/agp cards but nothing is going well. The processor is new celeronD 2.53ghz, 256 ddr is okay. I tried using different monitors but its still the same. It was working before. Help me what could be the problem guys.

I also have got this machine which is showing nothing but horizontal lines after playing FIFA 2004 game. Its an MSI board with 2.8 pentium processor, 256 ddr, onboard vga/sound/ network cards the monitor is 14" with resolution up to 1024*768........

HELP ME GUYS!!!!!!!!!!!!!!