Quote from Aderonius»Wrong section but it shouldn't happen. But it is always to back-up your important worlds by making a copy.
To install a map, it should be like this:
Download the map.Locate the .minecraft folder. The easiest way to do this is to open your Launcher, click 'Edit Profile' and then click 'Open Game Dir'. There is also another way, you can do that as well.Open the 'saves' folder.Copy the map folder (not .zip or .rar) in it.Play it from your list of worlds.If you didn't install it that way then there's your problem. If not, I don't know.