App drawer disappeared on One Plus One with Cyanogen OS
I had the same problem, what worked for me was accessing the Home Screen settings (on home screen, hit the menu button and then the three little dots towards the top of the menu, or hold anywhere on the screen and hit the same triple-dot symbol) and under "Drawer Settings," switching some settings.

I believe by changing some of these settings any problem causing the app launcher to disappear is corrected when it overwrites its properties.