The launcher is the app that draws your desktops and app drawer. I assume you are using the one that came with your phone, since you don't seem to know what it is, but different manufacturers have their own. Since you've not told us what phone you have we can't know what features your launcher has either, but some allow you to hide apps from the app drawer. So the first thing I would do is check any settings that relate to the desktops and app drawer and make sure that you don't just have a bunch of apps hidden. If many apps have disappeared it's probably something to do with the launcher, but stock launchers often don't have as many controls as replacement ones.
Secondly, can you confirm that the apps are missing from the app drawer, that they've not been grouped into folders (some launchers allow folders inside the app drawer), and that you can still place icons for the "missing" apps on the desktop? If you can't do that last then that will tell us that it's more than apps just having been hidden.
Sorry it's all questions, but need to figure out what's happening before can work out answers. The more information you can give us the quicker we'll be able to do this.