3 weeks ago I bought a two-way ticket Cologne-Istanbul-Cologne from Onur-Air. Yesterday I received an e-mail that the second flight got cancelled. Do I have a right to ask for a refund for both tickets and not just one? Has anyone else been in such a situation?

UPD: I called an operator and asked to get a refund for both tickets, that's what I needed the most and also I bought them together at once. The operator was kind and told that I will receive the refund in 3-5 days.