The ignition key turn all the way back, stuck?

Check fuses, check body grounds for damage. Also, is the park lock solenoid working properly? In other words, does the vehicle only come out of park if the brake is depressed?

For starters, reference this bulletin. See if it helps.
#07-07-30-032C: Shifter Binds, Ignition Key Sticks, Shift to Park Message Displayed On Driver Information Center (DIC), Dead Battery (Adjust Shift Linkage) - (Jan 8, 2010)

Subject: Shifter Binds, Ignition Key Sticks,"Shift to Park" Message Displayed On Driver Information Center (DIC), Dead Battery (Adjust Shift Linkage)

Models: 2007-2009 Cadillac STS-V, SRX

2007-2010 Cadillac STS

2006-2010 Cadillac CTS Sedan

2009-2010 Cadillac CTS-V

2010 Cadillac CTS Sport Wagon

Equipped with Automatic Transmission

This bulletin is being revised to add model years. Please discard Corporate Bulletin Number 07-07-30-032B (Section 07 -- Transmission/Transaxle).

Condition
Some customers may comment on one or more of the following conditions:

"Â The shifter binds when putting the vehicle into gear or park.

"Â The ignition key sticks or is hard to remove from the ignition lock cylinder.

"Â The "Shift to Park" message is constantly displayed on the Driver Information Center (DIC) when the shifter is in the "Park" position.

"Â A dead battery that follows after parking the vehicle for an extended period of time (less than 24 hours). This vehicle may have been left in accessory mode with the "Shift to Park" message displayed.

"Â The shifter binds when moving the lever from Drive to Manual mode.

Cause
This condition may be caused by the shift linkage adjustment, a slow Brake Transmission Shift Interlock (BTSI) mechanism or a Park Confirmation micro-switch that is not closed.

Correction
Adjust the automatic shift linkage following the steps below.

Verify the customer's concern.
Position the shift lever into the PARK position.
Check for any stored diagnostic trouble codes (DTCs). If any codes are set, diagnose the DTCs first using SI.
Raise and suitably support the vehicle. Refer to Lifting and Jacking the Vehicle in SI.
Adjust the automatic shift control linkage. Refer to Shift Control Linkage Adjustment in SI.
Lower the vehicle.
Tip
If the "Shift to Park" message is displayed, try wiggling or "bumping" the shifter while in the PARK position. If the message goes out, then repeat the adjustment. This is typically an indicator that the shift linkage is biased rearward. If the initial condition was Drive to Tap Mode binding, then the shifter linkage was initially biased forward.

Determine if the condition has been corrected. If the condition still exists, the automatic transmission shift control may need to be replaced. Refer to Automatic Transmission Control Replacement in SI.

Shift Control Linkage Adjustment
Position the shift lever in PARK.
Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle.
Notice: Refer to Fastener Notice in the Preface section.

Loosen the shift control linkage adjustment nut to allow the adjuster to slide freely.
Hold the selector lever on transmission against the rear stop to eliminate any play.
Tighten
Tighten the shift control linkage adjustment nut to 9 N Â m (80 lb in).

Lower the vehicle.

Inspect the operation of the starting system with the shift lever in each position. The engine should only crank when the lever is in the PARK or NEUTRAL position.
If the key is stuck or hard to remove from the lock cylinder or there is a high button effort, the linkage may not be adjusted properly. Re-adjust as necessary.

Check fuel pressure to start with.

May be the fuel pump loosing its prime. That would require replacing the pump.

Have the pressure checked when this happens

Roy

It could possibly be due to faulty ECM. Does the battery seem healthy, low voltage during crank may cause problems, as well as communication data line fault. It would need some hands on diagnosis to get an accurate idea, and this is going to be your cost variable.

Without saying you would need to have it checked for fault codes, try disconnecting both battery cables and touching them together for a minute or so. If you can't get them together, use a jumper wire. See if all the symptoms correct. If it resumes the same action, it will need some diagnosis, starting with a diagnostic systems check for fault codes.

Does the ignition rotate, car start? What is the security light doing, is it on solid? Try disconnecting both battery cables and touching them together for a minute or so. Restore power and see if functions return to normal.

Try putting a little pressur on your gear selector. Push it forwards (Towards park) and remove the key at the same time.

If this works, then you have a bent linkage/bad bushing in the gear selector mechanism.

If it doesn't work, then you possibly have something binding inside the ignition lock that is preventing it from turning completely to the "Off" position.

At this point I feel confident that is deals with the security system or the shift interlock solenoid. Something triggered it into lock down. It is odd that the neg battery terminal did that to you. Either someone over tightened it or it was left loose. However, that could have been the start of the entire problem.

It could be a battery that has shorted internally. I have seen this before.

Here are to guides to help you test and then replace the battery

https://www.2carpros.com/articles/car-battery-load-test

and

https://www.2carpros.com/articles/how-to-replace-a-car-battery

I have three things for you to try before the shop. First, disconnect the battery (neg terminal) for a couple minutes and then reconnect it to see if the system will reset itself. If that doesn't work, remove the neg terminal again then the pos. Hold them together (away from the battery) for about 30 seconds and then replace and see if that resets it. Finally, turn the key to the run position and let it sit (not running). The security light should go out after about 10 minutes (over riding the security system) and then start it and check if that helps so you can at least drive it to the shop.

Also check the shift lock fuse that could be out as well.

Let me know how things go for you.

Joe