It is an adverb that means "no longer". It is used when something happened or was true in the past but does not happen or is not true now.
You may hear "もはや" in lines from manga and anime. It sounds like a statement. "もう" is interchangeable with "もはや" and it is more often used in daily conversations.
Meg and Amy are catching up.
- Amy: How are things going with your boyfriend lately?
最近、彼氏とはどう？ - Meg: Didn't I tell you anything? I'm no longer interested in him.
何も言ってなかったけ？私、もう(orもはや)彼には興味がないの。 - Amy: What? You two used to be close.
どうして？仲良かったじゃん。 - Meg: We used to be. He cheated on me and made bad excuses. I can't trust him any longer!
前はね。浮気したあげくに、下手な言い訳をしたの。もはや(orもう)信じられない！
Meg is doing her makeup by Amy's blush brush.
- Meg: Wow, this blush brush from SEPHORA is good. It's easy to blend in cheek rouge with it!
わぁ、このセフォラのチークブラシ、いいね。チークを塗りやすい！ - Amy: Yeah, it's a good stuff. Since the brush is getting older, I want the new one. But they no longer produce it.
そうなの、これ、いいんだよね。ブラシが古くなってきたから、新しいのが欲しいんだけど、もう(orもはや)生産していない。
Ken and Leo are making a proposal in order to receive an order of the project.
- Ken: Can I go through the documents again before we submit it to our clients?
クライアントに提出する前に、もう一度資料を確認していい？ - Leo: What? I've just sent those documents to ABC company. There’s nothing we can do about the deal anymore. We can only wait for their response.
えっ？今ABC社に(資料を)送ったよ。もはや(orもう)僕達に出来る事はないよ。あとは、連絡(or回答)を待つだけさ。