iPhone 7 Plus Won't Turn On?
If a device seems like it's not responding to any input, often a hard reset, which is Sleep + Volume Down (for iPhone 7 and above; for iPhone 6s and below, use Sleep + Home) held until the Apple logo appears, is needed.

Powering off an iOS device isn't like powering off a computer. It doesn't reset everything, flush RAM, etc as a computer does when powered down. It's more akin to hibernation mode on a computer, as it's designed to come back on quickly. The hard reset is more like turning the computer off and back on - it dumps RAM and does any housekeeping that might be preventing the device from responding. It does NOT reset any data, preferences, apps, content, or so on.