Numpad keys not working

I am new to Blender and I have been trying to learn from a tutorial “Blender 3D D Design Course”, but for some reason my numpad keys are not able to change any views.
The only way I can get them to change is by using the “view” menu.
My system is Windows7 64 bit.
Thank you for any help!
PS. I have uninstalled Blender and reinstalled the latest version for my system, but still have the same problem.
Update
I have found that the numpad keys work perfectly in the Blender opening window.
But for some reason, in the “Yellow Submarine” exercise they stop working after I insert the UV background image of the submarine.