When I go to the store and want to install Whatsapp they say I already have the app installed but it doesn't appear in the app list. I run the preview for developers on a Lumia 620. How can I solve this problem?
Man, I'm sorry I couldn't help.
Unfortunately, the only thing I can suggest now is a hard reset. I would set it up as a new phone and redownload everything from your purchase history on the Windows Phone website. Wish I could have helped you more.