Lock appears on top of iPad screen

It's hard to tell from your description but you may be encountering the Activation Lock for iPhone, iPad, and iPod touch - Apple Support feature, related to iCloud - Find My - Apple . Activation Lock is designed to prevent anyone else from using an iPhone, iPad, or iPod touch if it's ever lost or stolen. Deactivation requires knowing the Apple ID and password that set the feature, or by placing a special request to Apple to reset the device. This Apple ID information essentially defines the current ownership of the device.

Click on this link to learn how to remove Activation Lock --> How to remove Activation Lock - Apple Support