unerwartet
unexpected
Describes something that happens surprisingly or that one did not expect.
Sein Besuch kam völlig unerwartet.
His visit came completely unexpectedly.

unexpected
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unexpected
Describes something that happens surprisingly or that one did not expect.
Sein Besuch kam völlig unerwartet.
His visit came completely unexpectedly.

The word consists of the prefix 'un-', which expresses negation, and 'erwartet' (expected), the past participle of 'erwarten' (to expect). So it literally means 'not expected'.
Wir bekamen eine unerwartete Rechnung.
We received an unexpected bill.
'Unerwartet' (unexpected) can stand as an adjective before a noun or as an adverb that describes a verb. As an adverb, it remains unchanged.
Das war ein unerwarteter Erfolg.
That was an unexpected success.
Common combinations are 'unerwarteter Besuch' (unexpected visit), 'unerwartete Kosten' (unexpected costs), or 'eine unerwartete Nachricht' (an unexpected message). It is often intensified by adverbs like 'völlig' (completely) or 'ganz' (quite/completely).
Durch den Sturm entstanden unerwartete Kosten für die Reparatur.
The storm caused unexpected repair costs.
once / times
'mal' in 'auf einmal' (all at once / suddenly) and 'unerwartet' both have to do with surprise. 'auf einmal' is a fixed adverbial expression and describes something happening suddenly. 'unerwartet' is an adjective or is used adverbially and says that one did not expect something. 'auf einmal' places more emphasis on the quick moment of change. 'unerwartet' places more emphasis on the fact that something was surprising. This is important here too: 'mal' by itself does not mean 'unerwartet'; only 'auf einmal' has this meaning.

Im Sommer kam die Krankheit unerwartet zurück.
In the summer, the illness unexpectedly returned.
In der Pause war meine Kollegin auf einmal verschwunden.
During the break, my colleague suddenly disappeared.