If you paid by credit card then you will have to fill out the claim forms from ATOL and they should refund you when you supply a copy of your c/c statement, it may take some time though so be prepared to wait.
If you paid by debit card the same applies but may take longer. If ATOL won't pay out then your c/c will refund you, if Atol won't pay out and you paid by debit card you have to go through ABTA