schlagartig
sudden / abrupt
Describes an event or change that happens very suddenly, abruptly, and unexpectedly.
Als das Licht ausging, wurde es schlagartig still im Raum.
When the light went out, the room became suddenly quiet.

sudden / abrupt
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sudden / abrupt
Describes an event or change that happens very suddenly, abruptly, and unexpectedly.
Als das Licht ausging, wurde es schlagartig still im Raum.
When the light went out, the room became suddenly quiet.

The word is made up of 'der Schlag' (the blow/strike) and the ending '-artig' (-like). So it describes something that happens 'in the manner of a blow'—that is, very suddenly and unexpectedly.
Die Stimmung änderte sich schlagartig, als die schlechte Nachricht kam.
The mood changed abruptly when the bad news arrived.
'Schlagartig' (suddenly/abruptly) is very often used as an adverb. It can appear at the beginning of a sentence, which requires inversion (verb before subject), or it can stand directly with the verb in the sentence.
Schlagartig verstand sie das Problem.
Suddenly, she understood the problem.
'Schlagartig' (suddenly/abruptly) often describes a sudden change in a state or perception. Common combinations are 'schlagartig still werden' (to become suddenly quiet), 'sich schlagartig ändern' (to change abruptly), or 'jemandem wird etwas schlagartig klar' (something suddenly becomes clear to someone).
Mir wurde schlagartig klar, dass ich meinen Schlüssel vergessen hatte.
It suddenly became clear to me that I had forgotten my key.
night
'über Nacht' (overnight) and 'schlagartig' (sudden / abrupt) both describe a very quick change. 'schlagartig' emphasizes that something happens very abruptly and in a single moment. 'über Nacht' means a change in a very short time, often from one day to the next, and sounds less focused on one exact moment. If something changes immediately, 'schlagartig' fits. If something has appeared or developed surprisingly quickly, 'über Nacht' often fits.

Nach der schlechten Nachricht wurde die Stimmung im Raum schlagartig ernst.
After the bad news, the mood in the room suddenly became serious.
Das Problem ist nicht über Nacht entstanden.
The problem did not arise overnight.