Airtime and data gone overnight

i checked my data and airtime balance last night and had R168 airtime and 400meg data. overnight all airtime and data is missing. how is it possible that my airtime can disappear overnight?! i did not go on the internet and did not use that much data. i want my airtime and data back. i do not believe that background apps can use that much data overnight. i have never had this problem. FIX IT NOW!!!