Battery

91 is a good % (start worrying when that value goes down close to 80%).

So the unexpected shutdown shouldn't be related to you battery, unless it has been a temporary power glitch, but we, just users like you, can't dignose that issue remotely, sorry.

Apple direct support can do it: try contacting them using this link:--> Get Support

Regards

Giulio

It could be a battery problem. Can you turn it on plugging it on charge?

Okay. Check:

Settings>Battery>Battery Health...and tap there. What % can you read?

As I said, if the issue (shutdown) persists, contact Apple Support online (click on thew following words, written in blue:--> Get Support

Regards

Giulio