my phone froze over night (i have a galaxy s7) and is on a black screen
Have you tried recovery mode? Usually, volume down and the power button will boot you into recovery. Other than that, there's plenty of ways to reboot your phone with adb. adb shell reboot -p 

Adb shell reboot-p  


will set you straight I believe

Edit: this will only work in the case of a frozen phone. If its off and won't turn on at all, and recovery mode won't work, it may be hardware related