Why did the plane turn black? I have added the texture correctly, but when I change the rotation on the X axis to 90° it becomes black.
It's most likely not untextured, just shadowed or backfacing.
Either rotate the plane 180º so the it's normal is facing outside or add a lamp to light that face up
You can check your normal on that particular face in the properties panel > mesh display in edit mode.
It should show up as a blue line facing outwards (you can also see vertex and edge normals).
If the normal is facing inward, you can sometimes see the face as black depending on your view.
If you have a cube mesh, you can select it, go to edit mode, select the black face only, go to mesh > normals > flip normal (w -> f), or you can just go to edit mode, select all with 'a', go to mesh > normals > recalculate normals outside (ctrl-n).
If you have a plane, just make sure the whole thing is selected and you can flip normal by pressing the w key, and then the n key (or making the menu selection flip normal).
Hope this helps!
ok, so in the properties menu, if you scroll down to shading, there will be a tickbox for shadless, tick it and it will render