Could it be the solenoid

your problem may be that the starter motor is bad. to test this you will need some help and a test light. there will be two wires that attach to the starter motor solienoid. the large one should have power all the time and the small one will have power just when the key is held in the crank position. if you have both of those it is a good chance that the motor has failed. use a volt meter to be shure that you have 12 volts poor connections at the battery . can also cause a simular problem.

Great answer from TNICK, I agree with his diagnostic and trouble shooting suggestion. Zee