Click

I have a 1987 mustang GT 5.0 H.O. With 129000 miles. The car was diving fine I parked it in the gaage and came out the next moring turn the key and nada. I tried jumper cables chec all the fuses and it still wont turn over I have power to everything but when I turn the key I get one click from under the hood. If you know what the prob is your help would be greatly appreciated