Without a screen shot [or SKP] we have to guess…
I suspect you have some faces ‘hidden’, when you switch hidden geometry ON you’ll see the face as a ‘grid’.
If that face is outside of the current edit context it is not selectable.
You can hide it using the edit > component edit > hide rest of model…
If you do not want it then select it and press ‘delete’.
To ‘hide’ objects it’s best to put them on Layers which are OFF.
Note that you should almost always put your raw geometry - edges and faces - on Layer0, then make groups or components and assign other layers to those. Layers control visibility, they do not separate geometry.
Hiding objects has its place when doing temporary operations, but usually using Layers is the better way.
Reserve ‘hide’ for making edges invisible etc, otherwise, hiding faces is a recipe for the kind of confusion you are experiencing…
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