How can i fix my Nissan almera 1.5L 05 plate (NATS) symbol on?

HI, i noticed today that when starting my car the NATS (anti theft system) symbol was flashing and the symbol stayed illuminated for the whole journey (5 miles).
I tried it again and same thing and then in the evening the car wouldn't even start and everytime i tried to turn the key over the NATS symbol flashed.
Its a nissan almera 2005 (05 plate) 1.5L.
If anyone has any ideas or has experienced similar problems please let me know. Thanks.