I just received an email stating the find my iPhone has been disabled

I just received an email stating the find my iPhone has been disabled and that anyone can get into my phone and use it. What do I do?

I received a message in messenger from my cousin whose account has been hacked. It said to download a blocker I downloaded it then deleted it but it shows up on apps downloaded how can I get rid of it?

[Re-Titled by Moderator]