Suddenly
Sudden
adverb, な adjective, の adjective
When something happens with a sudden nature, it happens... suddenly.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
突然、猫のお化けが台所に現れて、「ニャーニャーソング」を歌い始めたんだ。
Suddenly the ghost of a cat showed up in my kitchen and started singing the mew-mew song.
病院から突然、何百人もの赤毛の人々が出てきました。その中の一人に話しかけると、赤毛の人に対する大きな医学実験があったと言われました。
Hundreds of people with red hair came out of the hospital. When I talked to one of them, they told me there was a major medical study of red haired people.