Sounds like your battery has dropped a cell. If you wanna eliminate the starter then you can short it using a screw driver or run a jumper wire direct from a battery to the starter solenoid. If it cranks over its ya battery or a connection somewhere. If its a solenoid you can pull out starter and rebuild the solenoid cheaper than buying a reco or new starter. If you get it going get a multimeter set it to12v dc and connect it to your battery with engine running and make sure the voltage is going up

Use the car to diagnose itself.

Turn on headlights (which are a heavy current drain). Then try to start the car. If the lights dim, you are getting power to the starter. If they don't dim at all, you are not getting power to the starter.

If you are getting power to the starter, and still not starting, the starter might be bad or jammed. Or, the battery might be discharged and too weak to turn the starter but strong enough to operate other electrical systems in the car.

If you are not getting power to the starter, the ignition switch (neutral safety switch), or voltage regulator, or starter relay might be bad. You might also have dirty or corroded battery cables.

As lights etc work and won't jump start.. We either have immobiliser issue or starter itself.

As fuses and relays ok ..it points to solenoid failed. Bypass and test direct..if it cranks. It's solenoid . Starter normally supplied with new solenoid so exchange unit is probably best bet.

"Battery won't jumpstart either." I'm not sure what you mean by that but I'm wondering how you are connecting things when jumpstarting.

Otherwise, see hornchurch's answer, but remember that a flattish battery can power the lights but not necessarily the starter. Should still click though, if the solenoid was good.

The most obvious reason is that you have a dead battery that wont hold a charge.. If it wont jumpstart check battery terminal connections..remove. clean with fine sandpaper and replace tightly. if that doesn't work check the starter motor connections.

I was out of NZ for three weeks and was told that my car went flat, it was jump started while i was away and went fine. It has worked fine for the last 3 days. This morning nothing.

All the electrics are fine, radio, all lights, horn, indicators, dashboard, no flicker if I turn the key. No fuses have blown. No clicking sound or any underhood sound when I turn the key.

The battery wont jump start either. Battery looks clean and tidy though. Altenator cable replaced in last few months.

Battery, altenator or starter motor? Any thoughts? It is a Nissan Pathfinder turbo 2700 diesel 1991.