So do you say that the flight is actually happening, they just cancelled your ticket ?
Would be maybe good if you provided more details, like flight numbers and dates...
Also - if they cancelled the flight supposedly since you asked them for, it would make sense that you only got the tax back. But they should have a recording of the call which would then make it clear whether there is ground for the cancellation or not.
All that is best to discuss over phone, I think.