I had some issues with "Smart Key Not Found" and poor ket fob reception (only locking and locking when very close to the car) so I assumed the battery was getting low.

Whilst in the supermarket I saw the battery for the key fob so bought a new one. I changed the battery and now the key won't work.

None of the buttons responded with the car, so I had to open it manually. The alarm went off and so I tried to place the key under the steering column to be detected and start the car, but this didn't work.

The key just stopped working.

Any tips on why this might be, or how I can fix it?

 

I have had this issue as well. What I have found that works is to remove the fuse, I believe it's #17 a 5 amp fuse for at least a minute. Also disconnect the park assist module located in the rear compartment on the drivers side (left hand drive). There are three connectors. Once I put the fuse back in, my car is back to normal operation for a few weeks or months is some cases. Not sure if mine is a park assist issue but that seems to work for me.

Also once the car starts I reconnect all three connectors to the park assist module.

Good luck