once upon a time
1. used at the beginning of children's stories to mean 'a long time ago':
Once upon a time there was a beautiful young princess with long golden hair.

2. used in a slightly literary way when referring to something that happened in the past, particularly when expressing regret that it no longer happens:
Once upon a time people knew the difference between right and wrong, but nowadays nobody seems to care.

[TahlilGaran] Advanced Learner's Dictionary ▲

once upon a time
[literary]
1.
a long time ago.
Once upon a time there lived a young girl called Cinderella.

2.
if you say that something happened once upon a time, you mean that it happened in the past and you wish that it still happened now.
Once upon a time, everyone knew each other in this town and nobody bothered locking their doors.

[TahlilGaran] English Idioms Dictionary ▲

once upon a time
far in the past
Once upon a time a beautiful princess lived in a castle in a small town.

[TahlilGaran] English Idioms Dictionary ▲

once upon a time
adv. phr. Sometime before now, long ago.
Once upon a time she was thought to be the most talented actress in the country.
■ Often used at the beginning of fairy stories.
Once upon a time there lived a king who had an ugly daughter.

[TahlilGaran] English Idioms Dictionary ▲