What causes the iPad touch to suddenly stops working and what can I do?

Touch sensitivity is directly related to the screen "health". If it is broken, you need to have it replaced.

You can try a "forced" restart of your device, to see if, at least, you can re-gain the touch to create a backup before having your device serviced:

• On an iPad with a Home button: Press and hold the top (power) 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 (power) button. Only when the Apple logo appears (it might take 20-40 seconds), release that button (ignore all other screens that might appear).

If you get no help, start here:--> iPad Repair - Official Apple Support

Regards

Giulio

Not much.

You own a nearly 8-year old iPad that cannot upgrade beyond any version of iOS 12.

Your AppleCarre warranties have long run out.

Apple and Apple Stores do not do actual iPad repairs, but in/out of warranty device replacements.

A replacement iPad of your exact same model, if still available, will be expensive, costing approx. 60% of that old iPad’s original cost!

Even attempting to get that screen replacement from some third party electronics repair center, you need to find out if the repair center is competent in Apple products and iPads, in particular ( lots of iPhone repair places ) and that the electronics repair expertise is competent enough to do an actual iPad screen replacement using known working screens from other cannibalised screens from the same model iPad as yours.

No common, unauthoriswd, third party repair centers have access to any new OEM parts from Apple.

An actual screen repair of this iPad could exceed the actual worth of this iPad, which is not much.

Lots of variables here.

I would NOT put any type of significant money into this 8-year old iPad.

It isn't worth it as Apple stopped supporting this iPad model nearly two years, ago, AND third party app supoort is still continuing to dwindle for these 7-8 year old iPad models, today.

It's 2021 now!

It is time, now, to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.

There are PLENTY of newer, older iPad models that can run the latest, current versions of iPadOS.

I am sorry that you had to learn all of this, but this does not change any of the facts regarding these very old iOS devices.

Sorry & Best of Luck to You!