App deleted itself, can't re-install

Hi everyone,

I just spent 5 hours trying to fix this problem on my iPhone 4 (which was perfectly working until now) :

This morning I wanted to check whatsapp like everyday, but today some of my apps' logos were totally white (with grey geometrical figures, I don't know how to explain that), including Whatsapp that wouldn't open. I turned off and on my phone and these apps disappeared !! The camera was GONE, (even if I slide the screen to access to the camera, I wouldn't see the camera's logo) the Music app was also gone, Viber, Whatssap, and a few others that I don't really use anyway.

I first thought it could be because the phone was too full, but I can't even reinstall these apps (2.8 giga are cleared now), I deleted them (I mean "them", but actually I couldn't see the names) through the settings, and then tried to install them again... Didn't work, the app is downloading from the AppStore, but won't show on the screen and when the download is complete the error "Sorry, this app can't be installed right now" appears (more or less, my phone's settings are not in english).

I tried to install it with iTunes, but it doesn't work either and doesn't even show if anything is wrong, it's like iTunes believes whatsapp is installed but it is NOT on my phone. I've tried everything, I don't want to reset my phone, I've lost all my pictures, many of my friends are not in the countryI live, I need whatsapp to communicate with them, I'm a bit desperate here, so if anyone has a solution and an explanation, because I don't understand how and why these apps are gone without anyone deleting them.

Thank you guys 🙂