1.5 GB dissapears

I checked the balance on my Vodacom issued Wi-fi device on Tuesday (26/05/15) it was 1.56GB . I then switched it off and left it at home. On Saturday morning(30/05/15) I switch it on ,and connect to it, but I cant browse. I check the balance I now have 1.01MB left.. VODACOM why are you *****ing from me.. This does not make sense at all... How can 1.5GB of data just disappear.<br> I want my data back