The Battle.net desktop app logged you out because your account was logged in elsewhere. This can happen if you share a Battle.net account with family members or if you use multiple computers to log in from different locations.
Note: You will not be removed from games that are already running unless the new player tries playing the same game you're playing. Patching will also continue in the background despite this message.
If you suspect your account was hacked, visit our Account Hacked article for more information.