Why do I have a white bar across the top of my iPad

Can you post a screenshot - as with a better understanding of your issue, contributors here may be better able to offer appropriate help or guidance.

Take a screenshot on your iPad - Apple Support

You can post your screenshot using the image button at the bottom of the message editing window:

Otherwise, begin with a forced-restart of your iPad - as this will often resolve an isolated issue:

• 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.

If the problem remains unresolved, you may need to determine if you have a hardware or software issue. A reliable method to identity 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 bar is not present in the screenshot image, then you have demonstrated that you have a hardware fault that may require repair. If the bar to which you refer is 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.