What is that thing i cant Delete?

Hey, i was editing my Theater and this Square came out of nowhere and i cant select it, nor i cant find it in the Workspace.

How can i get rid of it? And what is it?

Thanks for your Help <3

It cant be selected and i tried to unlock it already

Got it, it was a MeshPart that didnt had the Data anymore to display the 3D Model anymore. In Case that happens to anyone check the Mesh you insterted because thats how it lookes like aparently if its not displayed correctly. Thanks for the hlep and the fast reply tho =)