No Data

I noticed my data was running low so yesterday afternoon I bought a data bundle of 500mb. I got an sms to confirm that the data had been purchased and it had been taken off of my airtime. This morning I got messages to say that my data was finished and the app also shows this. There is no way I used 500mb in one afternoon because my phone connects to Wi-Fi once I get home. What happened to the data that I had purchased.