abruptly vs of a sudden

abruptly and of a sudden both are adverbs.

Word	Noun	Adjective	Verb	Adverb
abruptly	No	No	No	Yes
of a sudden	No	No	No	Yes

As adverbs, abruptly and of a sudden are synonyms defined as:

Other synonyms of abruptly include all of a sudden, dead, on the spur of the moment, short, suddenly.

abruptly (adverb)	of a sudden (adverb)
quickly and without warning; happening unexpectedly; on impulse; without premeditation	quickly and without warning; happening unexpectedly; on impulse; without premeditation