Merged to prepaid, data disappeared

Yesterday our 2 GB data was depleted. 2 Gb almost gets us through the month. yesterday we topped up with 1 Gb of data. Our contract is being moved over to a prepaid contract today as the contract has come to an end. Now that it has moved over the 1 Gb of data that we purchased yesterday has disappeared. It seems that it has been lost in the migration over to prepaid. Vodacom claims that the data has been used up however we are disputing the loss of the 1 Gb of data as our activity on the computer was very limited after the purchase of the 1 Gb, background activities are also minimal on the laptop. We believe this data has been lost in the migration, please may you rectify this Vodacom?