When I turn the key, all I get is 1 click!

Check all battery connections and have the battery/charging system checked. Since there was a click it should rule out the neutral safety switch usually.

Turn the head light on. Are the Bright or dim?

Leave the head lights on and attempt to Crank. What do the head lights do (stay the same, dim a little, dim a lot, go out)?

Are you able to bar the motor over by hand? If not the motor may be hydro-locked. Consider taking #1 and #5 spark plugs out and inspect for any liquid. Now attempt to bar the motor over by hand.

Note, this sounds very much like the symptom when the intake manifold gasket near #1 gave out on my 200 GT. This allowed coolant to seep into the combustion chamber though the #1 intake valve. I was not able to bar the motor over by hand. When I took out the #1 spark plugs it was soaked with coolant. After removing the spark plug the motor could be bared over by hand. A need intake gasket later and back on the road.