It definitly can be the anti-theft...does the theft lamp blink while your trying to crank the engine...or stay illuminated?
The warning lamps illumianted entail a specific fault in each system...but if the THEFT lamp stays illumianted at all times...that means there's definitly a fault in the THEFT system...it's possible that it took until now to reveal itself. First try the procedure below...it's a password XXXXX for the anti-theft system.
Sorry for the delay, for some reason I didn't recieve the last posts...if the relearn didn't work, than there is a more serious problem with the anti-theft. The Telltale lamp is the THEFT lamp. Unfortunalty you can't purchase a scanner inexpensively for diagnostics on this, you need a factory level scan tool. The most common cause of this type of failure is the Passlock Sensor in the steering column, which signals the anti-theft system when the ignition key is turned on. You can purchase a sensor and attempt to install it your self, but without correct diagnostics it can be a number of items.
I left another post above, so see if you can read it as well...but you'll need to put the battery on a charger while doing this or put a freshly charged battery in, it cannot go low on charge while doing this - or the procedure will be unsuccessful. All the lamps on the dash should be illuminated, so it's the position just before you crank the engine. Let me know if your still having trouble.