Mods folder missing from .minecraft folder
Solution 1:
Vanilla Minecraft does not have a "mods" folder. Most likely case is that you need to install Forge Mod Loader, which you can download here.
If you do already have Forge and the folder is still missing, it may have been deleted by mistake. You should be able to create a new folder in the .minecraft directory, and call it "mods".
Solution 2:
If you got Forge and can't find the mods folder, you have to run the game at least once, then create a new profile on the launcher and for the game version select the one with Forge in the name.