Synonyms for All of a sudden:

adv

suddenly (adverb)
hastily, instantly, at once, immediately, precipitously, quickly, all at once, unexpectedly, suddenly, instantaneously, abruptly.

adv.

abruptly (adverb)
suddenly, unexpectedly, precipitously.
all (adverb)
all at once.
suddenly (adverb)
hastily, instantly, at once, immediately, precipitously, quickly, all at once, unexpectedly, suddenly, instantaneously, abruptly.

n.

all of a sudden (noun)
of a sudden, suddenly, all at once.

Other synonyms:

Other relevant words:
all at once, of a sudden, abruptly.