I hope I do not appear patronizing if I check that you have consulted the user manual that came with the car? If you have lost the manual then you can download PDF files for most Nissan models from here (www.courtesyparts.com/owners-manuals.php). If there is nothing in the user manual that came with the car (and there isn’t usually regarding resetting vehicle security systems), then you are left with two options to solve this problem.
The first option is the hands-on option of popping the hood and attempting to disconnect the wiring for the alarm from the battery, assuming that you have the standard Nissan Anti-Theft System which does not have a back-up battery source. If you don’t fancy messing around under the hood, you can try isolating the wires in the steering wheel column.
The one for the Anti-Theft System in Nissans is green and yellow but I urge you to carefully consult any wiring diagrams you have for the model of Nissan you own before trying this procedure. This should set the system to off. Reconnect the wires and using the correct key you should be able to set the Nissan Anti-Theft System in the usual manner.
The other option is a visit to your local Nissan dealer who should be able to reset your system using a scanning tool. This is the more expensive option but will guarantee peace of mind and shouldn’t affect your warranty, which messing around with the wiring of the car might.
As you can see, it does take a lot to disable an anti-theft system if you have access to the hood, so if you vehicle doesn’t have a specific lock for the hood, it may be worth investing in one and also a system that has a back-up power source and immobilization ability.