If it were me I would do the following
1. Find a screw driver and stink it in the hole and try and turn it a quarter of a turn.It sounds like you are missing the tumbler section of the lock.The screw driver takes the place of the tail from the tumbler and should turn the locking latch so you can open it.
2.If no success at that try a can of WD40 and spray it all around the inside of the hole (spray lots).You might have some rust around the locking latch and solenoid.This might free it up a bit
3. Have someone push down on the bootlid while you push the release button and take some of the pressure off the locking latch.
4. Last of all,,and this is VERY important,,,,join your local Cadillac LaSalle Club as you will find a wealth of information from guys who don't have a computer so don't see this board