いきなり, a word appearing almost exclusively in hiragana, is an adverb that conveys the meaning of 'suddenly', 'all of a sudden', or 'for no reason'.
いきなり indicates when something happens abruptly, and without any prior warning.
As
いきなり is an adverb, it will primarily appear at the beginning of sentences, or directly before the verb that it is describing.
いきなり電話（でんわ）してごめん。ちょっと相談（そうだん）したい事（こと）があるんだけど今（いま）時間（じかん）大丈夫（だいじょうぶ）？
Sorry for calling you suddenly. I need to talk to you about something, but is now a good time for you?
いきなり雨（あめ）が降（ふ）ってきたからびしょびしょだよ。
It suddenly started raining and I'm drenched.
ペットのワンちゃんがいきなり吠（ほ）え出（だ）したからびっくりして起（お）きた。
I was startled awake when my pet dog suddenly started barking.
Fun-fact - いきなり comes from 行（い）き成（な）り, but is rarely seen in its kanji form. This original meaning referred to a particular situation's development or unfolding. Due to this, although often being used purely as 'suddenly', it has a similar nuance to English phrases like 'a sudden turn of events'. This just expresses that the way the situation developed was abrupt or unexpected.