I was collecting some cars in my garage and it appeared to me that it was full, so it asked me to select a car to exchange it and I chose the Zentorno. I tried to get it back, please tell me how.
Go buy a larger garage and your cars shall appear.
You haven't lost any cars, you just can't access more than fit in your current garage. If you buy another 6-car or 10-car garage you will get access to all the cars again.
Possible duplicate of question linked above.