What to do if it's part of a mod I downloaded? All other textures work, except these aluminium stairs that are part of the chisel mod...they seem to share the same name with some that DO have textures though.
If it's a mod, you need to bring the problem to the mod maker's attention. Even though it is possible that the advice in this thread will help you fix it, there's no guarantee. Better to not spend your time, and just politely ask the mod maker to fix their work. Don't forget to post screenshots and session logs to help the mod creator out!
If you're using 1.11, the first thing to try would be to remove all of the capital letters from the file and folder names. Minecraft no longer accepts mixed-case characters so everything has to be strictly in lower-case.