Suddenly
All Of A Sudden
adverb
When something bears a sudden likeness, we say it happens suddenly, or all of a sudden.
Note that this word is relatively formal and somewhat literary.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼女が突如彼をセクハラで訴え始めたのには理由があります。
There is a reason for her sudden accusations of sexual harassment.