I assume you mean that the starter will not crank the engine.
The first thing that comes to mind is the nuetral safty switch that requires the transmission selector must be in neutral or park before the starter will engage. Try moving the selector to neutral and try starting the engine, or try jiggling the selector while attempting to crank the starter.
Next, I would open the hood and trace the heavy gauge battery cable that runs from the positive battery terminal to the starter. Look for loose connections or corrosion. Clean the connections and make sure they are tight.
If these steps don't solve the problem, see if you can manually turn the engine's crankshaft. Make sure the battery is disconnected, and use a large socket wrench to turn the nut on the crankshaft pulley. If you can't turn it at all, the engine is seized and requires major work.
If you can turn the crankshaft pulley, than I would suspect the starter has failed. Remove it and bench test it for proper operation.
If the strater operates on the bench, then there is probably a problem in the strater circuit, perhaps a failed ignition/starter switch.