I changed battery in 2006 trailblazer, truck started then shut down and went into security lock. key wont come out of ignition.

Mobile lock smith can reprogram key into computer , AAA'S has lock smith . Is the security light lit on the instrument cluster with key in the ignition lock cylinder . Car an padlock symbol .

check fuses relays and battery --lights going on is not an indication of battery being good --test battery with a meter or try boosting but your problem more than likely is key or ignition switch check --your key if worn or have chip key and chip/battery fail truck will not start if you have the other key try that one or try the relearn for the passlock system the link has information on how to do it
http://forums.trailvoy.com/showthread.php?t=69908

2006 Trailblazer won't start - Discussion on Topix

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Sep 6, 2012 - My 2006 Trailblazer will not start, bought a new battery, still will notstart, not even with a jump. ... i get a jump i drive it and charged the battery it seemed fine but then i went ... Then, next morning, when I turn key, all lights come on and radio... back a day and 360$ well that pos still will crank but will not start. 02 trailblazer a/c problem
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That is because of the vehicle theft deterrent system ! Do you have a security light lit on the instrument cluster ?
The Passlock?"¢ System is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate or the ignition switch is operated while separated from the ignition lock cylinder case. If starting is attempted without authorization from the Passlock?"¢ System, the powertrain control module (PCM) will disable engine starting. Start disable may be in the form of fuel disable or starter disable, depending on engine application.
The components of the Passlock?"¢ System are as follows:
?€¢
The ignition lock cylinder and key

?€¢
The ignition lock cylinder case, including the Passlock?"¢ sensor

?€¢
The ignition switch

?€¢
The body control module (BCM)

?€¢
The security indicator on the instrument cluster

?€¢
The powertrain control module (PCM)

Ignition Lock Cylinder Case, Including the Passlock?"¢ Sensor
The ignition lock cylinder fits inside the ignition lock cylinder case and operates the ignition switch when turned by a key with the proper mechanical cut. When the ignition key is used to turn the ignition lock cylinder to crank, start, a magnet on the lock cylinder passes close to the Passlock?"¢ sensor within the ignition lock cylinder case. The magnet activates the security hall effect sensor in the Passlock?"¢ sensor which completes a circuit from the security sensor signal circuit through a resistor to the security sensor low reference circuit. The resistance value will vary from vehicle to vehicle.
If a magnet from outside of the ignition lock cylinder case is used to attempt to steal the vehicle, the tamper hall effect sensor will be activated. This completes a circuit from the security sensor signal circuit through a tamper resistor to the security sensor low reference circuit bypassing the security resistor. If the ignition switch is forced to rotate without the correct key, or if the ignition lock cylinder is removed by force, the Passlock?"¢ sensor will be damaged and will not operate.

Your passlock security system has gone bad, you need to have your ignition key lock assembly replaced ! This is very common for this type of car and can be done with a lil time and a lot of patients.

check your battery might need a replacement and check also your alternator might be faulty and check electrical wiring from your battery,ignition going to your starter, check fuse and relay.

This mornnig when i tray to start my trailblazer 2006 LT sudnly eletiric power cuted and there is now light,now engen start ,the key can not poled out please rocomand me

Have the vehicle check for DTC'S diagnostic trouble code's . Qualified repair shop !

CHECK FOR BLOWN BRAKE LIGHT SWITCH FUSE OR SEE IF BRAKES LIGHTS WORK.IF YES HOLD FOOT ON BRAKE PEDAL MAKE SURE TRANSMISSION SHIFT ALL THE WAY IN PARK.IF ALL IS GOOD.SOUND LIKE KEYLOCK CYLINDER BROKEN.IF IT WAS SECURITY THEFT SHUT DOWN YOU WOULD SEE SECURITY LIGHT.AND THE KEY WOULD TURN BUT CAR WONT CRANK.JUMP START CAR AGAIN.IF BATTERY POWER LOW THE BRAKE TRANSMISSION SHIFT INTERLOCK NOT GOING TO ALLOW IGNITION KEY TURN IN SWITCH.

check the Ignition Starter Switch (tipical on tb)