Well it does sound kind of like the motherboard may have packed up, unless anyone else reading this thread has any other ideas ?
One other thing to try before writing off the mobo, disconnect the computer from the mains, hold in the power button for 10 seconds, remove the CMOS battery and leave it out overnight then replace it with a new battery (should be fairly cheap) and see if that makes a difference. If not then I guess a new motherboard is required.
If you want to have a go at replacing the motherboard yourself then have a go.
My advice would be to make a carefull note of where everything goes (take a few photos) and pay particular attention where all the cables connect to, especially the ones from the front panel
Once you have done that then have a practice run, remove your old motherboard and then re-fit it, it shouldnt take too long and will give you confidence when it comes to fitting the new one.
Remember when you come to fit the processor into the new board it will require cleaning and re-pasting with thermal compound.
Post back and let us know how you get on.
Learning all the time.