someone has signed into my apple account

No one can sign into your Apple Account unless the password is known to remove your Apple Account from your device. It cannot be done remotely. Are you seeing the correct Apple Account at these locations:

• Settings > [name on top]
• Settings > [name on top] > Media & Purchases

With a little more information, we may be able to provide specific information. At this point we don't know if you were talked into removing your account and signing another person's account into your iPad or if you purchased a used device that had not been properly reset.

Sorry but police has to find out or make a new apple account and report it to apple...

try to set up security on the account so nobody can steal your account or get your information

make sure you report them to apple