My Apple ID is locked but I didn’t do it. Does it mean someone somewhere was trying to hack in?

That is one possibility, but there could be other reasons.

Apple ID is locked or disabled

 Apple ID disabled - Your account has been disabled in the App Store and iTunes

If you have changed your password, have you followed the steps in this article?

What to do after you change your Apple ID or password - Apple Support     

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option. 

iTunes Support

iTunes Support – world wide

If there was a problem with your account in the App Store/iTunes, your ID could have been locked.

Not too long ago, my ID got locked when I was trying to access my account at the link below. I was the one trying to access the account and it was locked.

Apple ID - Manage