Ok i turned my PC on earlier an it wouldnt start up. I tried a restart and instead of helping it got worse. Now whenever the power cable is connected the fans and green power LED will come on. I try to hold down the power switch but it does nothing. I have tried disconnecting the hard drives, graphics card, RAM to pinpoint the problem and none of the previous have worked. Im thinking the problem is either the PSU or the MoBo. The current PSU i have is a 250W. Can someone help?

Is your monitor showing a message similar to "No signal" when it is turned on with the PC 'on'?
If it does then this would point to a problem relating to the gfx card and the monitor itself.
Otherwise what gfx card do you own, because if it is a high end gfx card, then your power supply is too small to support it. Just to check that your PC is booting correctly, remove your gfx card and plug the monitor into the VGA port on your mobo and check to see that the computer is booting normally. IF it doesn't, then it would point to a motherboard issue.

Ok i tried* it still doesnt boot properly. The Graphics card im using at tha moment is a nvdia geforce FX5600 DVI SVGA.

This sounds like the CPU is not recognised by the motherboard, but it does detect there's a CPU there.

If it's a new CPU it likely means it's too new for the motherboard (but maybe flashing the BIOS will solve it), if it's an old machine the CPU may be damaged and need replacement (which may also mean replacing the motherboard if you can't find a CPU that fits it and is recognised by it).