Why won't my phone turn on or off

[AC Question]

And it says downloading don't turn off target

 

[B. Diddy]

Welcome to Android Central! Which Samsung phone is it? You're in Download (Odin) Mode, which is usually reached by powering on with the Power, Home, and Vol Down buttons all pressed at the same time. As long as you're not connected to a computer via USB, you can force a shutdown by pulling the battery (if possible) or by pressing and holding Power and Vol Down for about 10-15 seconds. Then power on again normally.