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