iPad new lock icon — NOT solely related to screen orientation

A new icon has recently appeared on my iPad home screen: it’s a black square with, inside it, a circle [not an arrow circle] with, inside that, an open padlock. This new icon can be moved freely around my Home Screen . When tapped, the padlock “closes”, and my home screen becomes completely frozen: not only can I not change the screen’s orientation, but I’m also unable to swipe left or right to call up alternative screens, open any apps, and a long press does not open the screen to being edited – it is simply frozen.

Any ideas on why this might have happened, and how I can get rid of this annoyance? Many thanks for any help or suggestions.