An easy fix? Change your password from your new computer or new location. This requires you to authenticate with Blizzard -- and answer your secret question -- so the servers are sure that you're actually you. Another helpful tidbit: have an authenticator (or authenticator app) attached to your account and SMS protect enabled. These helpful security features not only make it harder for your account to get stolen -- they can make it clear to Blizzard that your account is in your hands, even if you're logging on from a new computer.
For more details and tips, check out Araxom's thread on Reddit. And if your account actually has been hacked, it's going to take a few more steps, but we'll walk you through it.