I tryed starting my 2006 cadillac cts this morning and the key turns over and the car wont start. also as i bring the key back the key wont come out of the ignition. now its stuck. my key fab does not unlock or lock my car anymore. i disconnected the battery and tryed boosting it. that never help. tryed moving stearing wheel back and forth never worked and also tryed pushing in brake and moving from park to N R D nothing.

push on brake pedal and move the shifter park neutral. Into park and try to remove the key now.

This link had your same problem last comment may help
https://www.cadillacforums.com/forums/cadillac-cts-first-generation-forum-2003/414449-cadillac-starting-problem-after-serpentine-belt.html
This link had your problem and found a solution
https://www.cadillacforums.com/forums/cadillac-cts-first-generation-forum-2003/890066-car-wont-start-after-replacing-serpentine.html

check key decoding or try spare key

Disconnect the battery and clean both terminals. Wait about 10 minutes before re-connecting battery terminals. That will reset your car's computer. Should start if there is enough juice in the battery. If you hear a clicking sound when you turn on the engine it means your battery is too low to turn the engine and probably means it's time for a new battery. Get someone to give you a boost and it should start fine. Your battery is probably original equipment and more than 8 years old. Thats about maximum life expectancy for a battery.

Have you tried to jump the battery and crank it ?
Its possible the alternator failed or something happened to drain the battery.
Is the security warning light on ?

There is a magnetic switch on the 'park' position. The car only starts or releases the key in 'park' is enabled. Maybe this switch is not working! Try to replace it!

i have a 2006 cadillac cts and when i get gas it dont wont to start ,,i haft to keep trying to start it but when it does ,,it runs like a new one ,, help please,,

sounds like you need to bring the car into caddilac to have the passlock systen (security) reset, these cars can be very sensitive to battery loss

2005 CTS 3.6 Dead battery, key stuck, service anti theft and nothing happens when you turn key, Ttried everything, ECM reset, key memory reset, charged battery to 12.7 volts, locked door w remote and opened w key, anyway you get the idea. After 3 days of online reading and trying, THIS IS WHAT WORKED, 1. put key into on position 2. removed sarter relay from front fuse panel 3. jumped 1st pin closest to fender and passenger seat with pin that is closest to motor and front fender. I used a paper clip and just made contact long enough for car to start. ran car for five minutes and took key out and everything went back to normal. then I took car to advance auto parts to have engine light reset. if car does not go back to normal you have ECM (car computer) failure. video on YouTube under "no crank, no start"

Have the TPS checked out first