[AC Question]

My S4 was downloading for over 8 hours with the robot and telling me not to turn off. It was obviously stuck. I pressed the power button and volume up button. This allowed the device to play the power on music and then it went blank. I can now hold the power and volume up button and see the Galaxy S4 logo just for a second and then it powers off.

[B. Diddy]

Welcome to Android Central! First, the Download Mode is how you can copy a different firmware file to the phone. It's not dangerous to power down in this mode, as long as you're not connected via USB to a computer and actively transferring a firmware file. You typically get into Download Mode by powering on with Power, Home, and Vol Down pressed at the same time.

Second, it may be that the battery drained very low while you get the phone one during that time. Let the phone charge on a wall outlet for at least 4-6 hours before trying to turn it on again. If it still doesn't work, see if a new or different battery works.