Blocked ignition switch

Fuel, air, spark, compression, valve stuck, head gasket etc etc etc
Be more specific

Since it is an older vehicle I would suggest starting at your local junk yard. Call them up and tell them what part you need, they should be able to tell you whether they have one on hand or not. Otherwise...I hate to say it, Ebay, I was able to find one with a quick search. (http://www.ebay.com/itm/NISSAN-240-SX-CONVERTIBLE-MOTOR-AND-PUMP-OEM-/161131942310).

If the battery is holding a charge, I would look at the alternator.

Most newer models have a safety feature called
shift interlock. This prevents the key from being turned back
to the lock position if you forget to place the gear selector in park.
Depending on the model,
If it uses a cable, it may be sticking or needs adjusting.
If it uses an electrical solenoid. may be blown fuse or park switch.
Tap, shake, and pull up on the shifter and make sure it's in park.

I don't know very much about cars myself, but I had the same problem with my 96 Nissan Maxima. It turned out that the button on the shifter knob wasn't releasing all the way, causing the key to be stuck in the ignition. I pushed on the button a few times to get it unstuck and the key came right out. Hope this helps.

Have you tried to turn the wheel far to the left as you try to turn the key? Also try to turn the key backwards in the ignition while holding wheel to the left. If those don't work, maybe try sliding the key in with a little WD-40 as this will lubricate the ribs in the ignition slightly and just insert/remove key slowly and quickly a few times and then try the steps above.

Does the car have a key release button? Sometimes these get stuck with the wheel turned slightly and the ribs of the ignition will not close on the key to allow for operation of the ignition.

Let me know,

Chip

sounds as the fuel has gummed up in the system, next time start your car regularly, get a can of starting fluid, spray it where the air filter is, may have black tube going to the fuel injection, you may need to dissemble few parts to do this,Once you get it started let it run for an hour.

Gear shift mechanism, there is a recall out on the parts I've had mine replaced twice already

There has to be 12 volts in system for the key to release its electrical for it to release

check the fuse for the cruise control (CC). Secondly, use a meter to make sure the switch is good underneath the brake lever. There are typically two switches underneat the arm; one controls your brake lights and the other controls the on/off [cancel] feature of the CC.