I built my pc about 6 months ago now and until recently it was working perfectly. However today when I tried turning my computer on the power button didn't work. So I opened up my computer and pressed the button on the motherboard to start the computer: everything works perfectly. Next I unplug everything from my motherboard and plug it back in, and then try to power on the computer through the start button, nothing happens. I'm very confused as of what to do here or what could be causing the problem. Is this a problem with my motherboard? Any other ideas?
If it powers on and works fine when you use the button on the mobo, then most likely the Power switch or its cable is broken. Trace the wires to the front panel switch and see if one has broken off, soldering it back on will fix it. You may not be able to replace the switch assembly, but you could drill (or melt) a new hole for a momentary-contact pushbutton switch and use that. You could use the Reset switch as the Power switch by exchanging the cables, but of course then you lose the Reset function until you've replaced the switch.
It sounds like the case power button is no longer working; test it further by connecting the reset lead on the front panel to the power lead on the board, then press the reset button on the case - if that starts your computer, the case power button is not working. You can try ordering a replacement.