Pavlov's dog salivates each time he hears a bell. Now, however, after several trials of salivating to the bell and NOT receiving any food, the dog stops salivating. What happened?

Lack of the  stimulus the dog stops to respond.

Explanation:

This an example of the classical conditioning as the dog salivates each time he hears the bell and when the dog did not receive the food the condition was broken and no response was given by the dog.