8ta cap disappearing

Hi there, I purchased 3gigs of extra data on Saturday evening for my husband to download something for work. Before going to bed i checked the data and we were sitting on 1.5gigs left. We were out the entire Sunday and came home in the evening to discover no internet. We have passwords and can monitor who is on our network and no one has connected outside our devices. I know there have been many many complaints about 8ta data disappearing and we have experienced this before. Please advise how you will be rectifying our missing data!