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With these in mind an drove fine yesterday which one would you try first??
As others have said BATTERY. Even though it drove fine yesterday, if a cell goes bad, the result can be instantaneous. Check the cables on the battery. If tight, take the battery to AutoZone or some place similar and have it load tested.
The order I listed them in is the order I would suspect the cause most likely is.
Easiest to check is the battery since you can remove it and have it tested free of charge. If you have and know how to use a digital multi meter the battery voltage across the battery posts should be 12.7 to 12.9 volts or more for a good battery at full charge. Have the battery load tested at any parts store and most do load testing free.
Also the pellet in the key not being read correctly is easy to check. As I said above if the SECURITY light on the dash remains ON with the key left in the ON (not start) position for say 15 seconds, the key pellet or the sensing rollers in the key cylinder are the problem. Correct operation of the SECURITY light is that when you insert the key into the ignition and turn the key to ON the security light should come on and then quickly go OUT.
If the problem is the starter or starter solenoid you can also check that out fairly easily. Place the end of a broom handle on the body of the starter (not the solenoid) and while a helper holds the key in the start position give the other end of the broom handle several very hard wacks with a 3lb hammer. No love taps. Solid wacks.... If the engine immediately cranks then the problem is the starter or starter solenoid.
Testing the ignition switch is more difficult. Here is a link to the procedure that Bill Curlee, Forum's resident electrical wizard, recommends: