My iPad is frozen with this popup message on the screen: "This accessory may not be supported." What can I do?

See >>> If you see an ‘Accessory may not be supported’ alert - Apple Support

If you are using a Smart (folio) keyboard, the alert is typical of a faulty folio keyboard. The electrical connections that pass though the flexible fabric hinges typically fracture - this usually resulting in errors and/or the on-screen notification that you see.  If you have an attached keyboard, disconnect and force-restart your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Note:  If iPad still doesn’t turn on, or if it gets stuck during start up, see the Apple Support article If your iPhone, iPad, or iPod touch won’t turn on or is frozen. Or if your iPad isn’t working correctly after you restart it, see the iPad Support website.

The Smart folio keyboard itself is not repairable. The only remedy for a defective keyboard its is replacement. If you are within your one-year limited warranty, you’ll need to contact Apple Support or your retailer.

For Apple Support, you’ll find the Get Support link at top-right of this page.  Alternatively, you can initiate a support case using the excellent Apple Support App. If not already installed on your device, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

A further alternative is to visit the Genius Bar at your local Apple Store, where the technicians will be delighted to assist. When visiting the Genius Bar, a reservation is recommended:

Genius Bar Reservation and Apple Support Options - Apple

What were you doing when you first received this message? Were you trying to use the charger or trying to connect to bluetooth speakers? My suggestion is this - force a restart by following the directions in this Support link: Restart your iPad - Apple Support

If you cannot restart your iPad, then see the link at the bottom of the page regarding - If your iPad won't turn on or is frozen - Apple Support

Hope this helps.

Read and follow the article linked above.