Welcome to Android Central! The native brightness adjustment should never let the screen go down to completely dark -- but if you have a 3rd party screen dimmer app installed, that can happen. I agree with mustang7757, try rebooting. Since you can't see the screen, the best way to do that would be to force a shutdown by pressing and holding Power for about 20 seconds (if this is a Samsung phone, then hold Power and Vol Down for 20 seconds), then waiting a few seconds, then powering on normally.