How can i open the trunk if the it will no open with the key or from botton inside?

Battery may be dying in remote and your car should have a setup menu which is accessed through the mileage display. try looking there, also refer to your owners manual if you still have it

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A lock smith may be able to pick the lock without damaging it, then the lock can be removed and he can cut you a new key.

I would check the owners manual or with Cadillac - its possible the remotes lost part of the signal and need to be reset. It sounds like the receiver is not getting a signal from them by using that button.
Could be the receiver too.

1. Open trunk
2. Place latch in lock position
3. Pull inside trunk release to see what isn't moving
4. If trunk key won't turn, the key may be worn out and need to be replaced

will it not open or stay shut?

Isn't there botton somewere on the door or left lower conner of dashboard to open trank remotely?

try removing rear seats to obtain access

there is a valet switch which prevents this check the glovebox or the panel that opens with the adjustment controls not sure of your model year has all this but if it does you would know where it is

inside the trunk is an emergeny pull cord. when u turn the key there is a little piece on that cord that the key pulls on... u can only fix it with the trunk open though. so to open it get the remote or have a second person to open the trunk right after u press the botton. open it while the latch is rewinding

Look in the glove box and see if there is not a valet switch. It is usually on the top left. This locks out the buttons.