Something happened when editing a Blender scene that caused me to lose the whole texture properties window and most of the materials window as well. They all appear as normal when starting a new file, but for this file they seem to be permanently gone.
This is what the window looks like now:
When using the Cycles render engine, the textures tab will only show if you have a texture added to your material.
If there's no texture properties, then you can enable it by assigning a texture of any kind to the color input of your shader.
It seems you changed the rendering engine to Cycles accidentally. In the Info window (aka the main menu), click the drop-down list which reads “Cycles Render” and change it to “Blender Render” (or “Blender Game” if you are using the Blender game engine).
There is also a bug that causes the texture tab to disappear, regardless of whether or not you have a texture applied. You must restart blender to have it reappear.
