Can't Find the reference number

I made a chnge on a booking on the 08 April 2015 and the traveller was suppose to travel on the 09 April 2015. I was told that they will call me back with the cancellation number and the lady I was talking too never called me back. I now want to use the cancellation number to book the ticket and I just called the Greyhound office and they are telling me that they can not pick up the booking. I asked the person that was helping me, what am I suppose to do since they can't find it.He said I must go to the Greyhound offices.Who must pay for that......I need the mater sorted