Please help , I dont want it to beat me?

Hi this is an update on an earlier question I asked.......

Hi , can anybody help please? I had a external hard drive connected to my pc via usb which shorted out, my monitor froze & had to switch off by mains switch. I tried to restart it but didnt get the usual quick beep on start up and the monitor displayed no signal, though the fan was on and the dvd opened & closed.... I have sinse installed a new psu , motherboard (same as old 1, make and model) and still no difference. The system starts up for approx 10 seconds then turns its self off, plus it still comes up with no signal on the monitor, I have also put an old graphics card back in that I did have installed in the pc before, still no joy, could it be the processor that is the problem ? I dont know what else to do, I'm getting to the stage of chucking it but I dont want it to beat us. Please help me to beat the pc. Sorry for making this near on a book