Issue Description
A transparent red box obscuring your model has appeared in 3D, and you are unable to turn this off, select it, or delete it.
Solution
This transparent red box is known as the trimming body and will show under any element that can produce a trim, such as a roof. It's used as a guide to show the extent to which the trim can take effect.
To see these trimming bodies while you work (in the 3D window only), turn on Trimming Bodies in the View > On-Screen View Options menu.