If this is the Factory Alarm, you should be able to DisArm by turning the Driver or Passenger Door Key Cylinder to Unlock or Unlocking Doors with Factory Remote.
If not there may be a problem with the Security System.
You also can try:
Be sure the Battery is fully charged.
Close Hood & Lock all Doors.Leave vehicle "undisturbed" for apprx 30 minutes.Unlock Doors with Remote or Key.Start vehicle.One other way, Disconnect Positive Battery Cable.Open Driver Door & LEAVE Open.Turn Ignition to ON (run) position & LEAVE On.Reconnect Positive Battery Cable.Alarm may sound for a few seconds.Start vehicle
If none of the above work, you may need to have the Security System checked by the Dealer