Important

If that line appears on ALL screens (even while using apps), it might be a hardware (screen?) problem, but we, just users like you, cannot diagnose that kind of issue remotely.

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.

Begin with a forced-restart of your iPad - as this will help to ensure that an isolated software issue is cleared.

• 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.

Next, a reliable method to confirm a screen or other hardware fault is to take a screenshot when the fault is evident - and then review the captured screenshot image...

When reviewed, if the lines are not present in the screenshot image, then you have demonstrated that you have a hardware fault that, sooner or later, may become sufficiently severe that repair or replacement of the iPad are the only options. If the lines are visible in the screenshot, the something other than hardware will be the cause.

Note: When reviewing the image, rotate the iPad screen orientation by 90°, from Portrait to Landscape, to ensure that the image does not coincide with screen defects if present.

Take a screenshot on your iPad – Apple Support

If this test shows that you have a screen hardware fault, your iPad will need repair. Your best option would be to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple

Alternatively, you can use Apple’s mail-in repair service:

iPad Repair - Official Apple Support