Hello,
So I have a troubleshoot as well as a workaround.
1.) Go to your world outliner and make sure that the camera has been deleted from there.
2.) Add a new camera to where you want it to record.
3.) With that camera selected go to your matinee window.
4.) If you still have the track you created with the old camera you can right click on that camera group and go to actors. That will have another small window that pops up and you can add selected actor to that group. That camera will then follow the old groups key frames.
5.) If this does not work or if this is not what you need what you can try is to go into your projects directory while you have your project open. You may still have your camera as a .uasset inside of your directory. Delete it and save all in your project.
Let me know if this helps,