Having an extrusion UV Issue

I am trying to model an item for an animation I am doing. It is a Minecraft animation, so I just put the texture, cleaned it up, and when it came time to extrude it to make it go from 2D - 3D, it has a weird texture glitch like it is transparent. I’ve tried everything, even restarting the whole process. I am fairly new to blender, and it is probably some simple issue that passed over my head, but I have looked and searched for a solution and could find nothing. Any help would be greatly appreciated!

HI, the texturing is always done after modelling is complete.
and you probably will need to unwrap your model first.