The flight was cancelled: am I entitled to a voucher or a refund?

But it depends on who cancelled the flight? Because the answer changes quite a bit.

If the airline cancelled it (overbooking, technical issues, staff strikes), then you're entitled to a cash refund, not a voucher. This applies in Europe under EC regulation 261/2004. You can choose between getting your money back, an alternative flight on the same day or one in the following days, or a partial refund if you accept a flight with fewer destinations. The airline can't force you to take a voucher: a cash refund is your right.

It's different if the cancellation was due to extraordinary circumstances (extreme weather, security issues, situations like what we experienced globally a few years ago). In that case the airline might be exempt from paying a refund, though they still have to offer you an alternative flight at no extra cost. But even then, a voucher isn't mandatory if you ask for cash.

In practice: first check the reason for the cancellation (it'll be in the message they sent you), then contact the airline and make clear that you want a cash refund, not a voucher. Keep all your proof of payment, emails, messages. If they push you on the voucher, remind them that it's not legal for a cancellation on their end. It might take a while to get your refund, but it's your right.