spinning wheel and dark screen

I can’t say what caused it. I have had my iPads and iPhones shut off out of the blue on me over the years. The forced restart always worked for me if the devices got stuck restarting or wouldn’t start up with just the power key.

Try a forced restart. Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo.