Repayment cancelled with no explanation

I submitted my return on the 6th April and was told I was due a refund so put the request in. I then chased this on the 13th May and after speaking to an an agent and conforming the numbers was told it would be paid within 10 days and it showed as repayment sent on the 15th May. Then the repayment disappeared completely and I didn't receive the refund. I spoke to HMRC today and they said the repayment was rejected on the 14th may and cancelled on the 26th and no one can tell me why? Just that the refund isn't due? I know my numbers are correct, and it's the HMRC system that has told me I'm owed something, but I have been told I can't speak to anyone about this and have to appeal. How can I appeal if I don't know why HMRC feel I'm not owed anything? I have been out of work for most of the last tax year and this money is the difference between keeping or losing my house so it's quite important. Why can't I speak to anyone?

Hi,

We are sorry you did not receive an explanation as to why your refund was cancelled when you last contacted us. If you have been advised to appeal against this decision in writing, please write to us and we should be able to give more detail as to why we had to cancel your repayment:

Self Assessment,
HMRC,
BX9 1AS

Thank you.