That happened to me once on a BA flight London to New York, some years ago. All was smooth, we got to about Cornwall and suddenly the plane seemed to drop through the air, then the engines roared and lots of people were screaming. I spent the rest of the flight in a state of high anxiety not helped by no explanation or calming words from crew or captain. Clear air turbulence apparently.
I used to type 'near miss' reports for the airline I worked for. I never realised how many there were. I didn't like flying much anyway, but it didn't help!