Imessage and app store jot working

Sometimes the imessage app and app store app half stop working.

Imessage doesnt let me log in

App store works except i cant download anything - says cannot connect to itunes store

Its wierd cuz every othere app ive tried works like safari and my games

Please help try and tell me how to stop it permanantly ITS SOOOOOOO ANOYING!!!!!!!!!