How do I get out of "Select Boot Mode"?

I accidentally got into "Select Boot Mode" screen setting and can't get out of it. I tried the volume up & volume down technique shown on YouTube, but it doesn't work. This has been on my screen for 8 hours. I can't turn off the phone or change the setting in any way. I can't use the phone and am concerned that this will burn the screen. the ATT store doesn't open for another 14 hours and I am hoping this doesn't ruin the phone. Any suggestions?