well there is a Neutral safety switch which only lets the car start when the clutch is depressed.
was the clutch in when you tried to start in N?
i would clean the battery connections.
this and a weak battery are the leading cause of a no start issue.
ESPECIALLY if you get a click.
the click means it is getting juice, just not enough.
another possible cause is the contacts in the starter solenoid.
they get old, charred and worn and do not conduct as they should.
the parts are cheap ($15) and it isn't a very hard job.
there is a great write up on this with pics and a source for the parts.
you can test for this cause by jumping 12volts directly from the battery to the small spade connector on the starter solenoid when the key will not start the engine.
if the jumper wire starts the engine then the problem is in the solenoid, or maybe the starter.
but the starter motors are pretty strong and don't go bad as often as the contacts, imho.
you could buy a reman-starter, but with new contacts and plunger in the solenoid you can reman your own.
so battery connections first, then solenoid contacts if needed.
Metro Heavy-Duty Distributors, Inc
1540 Plantation Road
Roanoke, VA 24012
1 (800) 296-8995
SOL CONTACT ND
SOL CONTACT ND
PLUNGER ASSY ND-GR
A quick Google search (which is where I got the above photos) reveals these to be, presumably, Denso replacement parts. If you search these part numbers you get several options for sites from which you can order them. Hope this helps anyone looking around, since the one thing I did not see on other threads about this issue were actual part numbers. Cheers!