Whatsapp application disappeared One minute i was using whatsapp, next minute it was gone, by itself.

I can see whatsapp shorcut on homescreen but when i tap on it says "the application is not installed". I checked the installed applications list and it is not there.

When i try to re-install again it says "unknown error occured during installation. error code -24". 

Can't run it, can't re-install it. Wow, what a weird error! Though i am not surprised since fb acquired it:P
If your phone is rooted and you use a custom recovery (like TWRP or CWM)Try recovery mode and wiping dalvik cache and fixing permissions and reboot.