Lost Samsung Galaxy S6 help!
If the device has been taken or lost and is off, there's not much you can do.

If you think it may have been switched on, there is a section on your Google Account that will allow you to track/lock/call/sign out of your phone. More options can be explored on Google's help page for lost devices.

There's also a setting to Ring the device when you select Locate from the last link, which will ring the device at full volume for five minutes, even if it was set to silent.