all of a sudden / all of the sudden
The idiom for something that happens abruptly.
All of a sudden is an adverbial phrase meaning suddenly. It preserves an old noun use of sudden, “an unexpected occurrence,” which also survives in the older of a sudden. All of the sudden is a casualism, common in speech, and does not belong in standard English. When the phrase adds nothing, suddenly says the same thing in one word.
Examples
Use The kettle sat quiet for ten minutes and then, all of a sudden, began to shriek.
Avoid The kettle sat quiet for ten minutes and then, all of the sudden, began to shriek. (Change the to a.)
Use The engine suddenly stalled halfway across the bridge.