After iOS update iPad won’t reboot

If you already tried (to no avail) all the steps provided at the section "If your screen is black or frozen" in this support article:--> If your iPad won't turn on or is frozen - Apple Support, I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

I have just installed ios 15.3.1 on my iPad. It died during the

download, and will not respond to any reboot method. iTunes will not

restore it, I get a failed message with no reason other than (9),

Nothing will reboot it - any suggestion re 15.3.1?

Well, even my Airpad2 was not working. I wanted to download apk files from a website but the next moment it got an update and starts performing normally. I don't know how it happened.

To mt knowledge "apk" apps are for Android smartphones, not for iOS/iPadOS devices.