Black line

Without seeing a screenshot of precisely what you can see, it’s difficult to guess or provide a definitive answer.

You can easily determine if you have a hardware or software issue. 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, rotating the iPad screen orientation by 90°, from Portrait to Landscape, will ensure that the image does not coincide with screen defects if present.

Take a screenshot on your iPad – Apple Support

If the test indicates that a hardware fault (as, given your description this is very likely) a visit to the Genius Bar at your local Apple Store is recommended - where the technicians will be able to test and assess your iPad. This will likely inform your next steps:

Genius Bar Reservation and Apple Support Options - Apple

iPad Repair – Official Apple Support

Should you wish to do so, you can post your screenshot here. Before posting a screenshot, ensure that you have first redacted - using markup tools - any personal or sensitive information.