Lost data

December 30 my data depleted l topped up with 100gig and on the 6th MTN topped me up with a recurring data of 70 gig then on the 30th of Jan midnight all data was depleted now they are failing to give me an explanation as to where my data went to since last Tuesday. I would have understood if the data that depleted was the one that l bought in December. Since last Tuesday lm dataless n they keep telling me to call after 24 hours but no explanation then one of they dats they topped me with 20gig YouTube, for what because l need my data not the YouTube promotional data . Kindly help.