unexpectedly -  неожиданно, как снег на голову

They  arrived unexpectedly.

The plane was unexpectedly delayed.

And some synonyms:

suddenly

A pedestrian suddenly appeared in front of the taxi.

all of a sudden

It seemed to happen all of a sudden – the electricity turned off and all my work ended.

out of the blue

One day, out of the blue, my manager announced that she was leaving.

out of nowhere

Suddenly, out of nowhere, she offered me the job.

without warning

Completely without warning, he turned up at my door with all four children!

without prior warning

The shooting came without any prior warning.

And two my favourite:

like a bolt from the blue  - как обухом по голове; как с неба свалиться; будто снег на голову

The resignation of the chairman came like a bolt from the blue.

out of the blue

One day, out of the blue, my manager announced that she was leaving.

#synonyms