all of a sudden

all of a sudden (something that is done unexpectedly) — вдруг, ни с того ни с сего; внезапно; неожиданно; ~ как гром среди ясного неба

Example: "And all of a sudden she jumped up and ran out the door without so much as a goodbye."

see also
[как гром среди ясного неба]
[out of the blue]