Stuck in "This accessory may not be supported." Can't restart.

My iPhone 7 has this error message:

"This accessory may not be supported."

I tap Dismiss, but the message doesn't go away. With or without a charging cable connected, the message still won't go away. I can't do anything else with the phone. I can't even restart it or shut it down.

Is there a way out of this? Thank you!