I booked while the nightly rate was discounted on the hotel's/agency's website, but they called to say I can't stay at that price. What can I do?

What matters here is how plausible the price was. As of 2025, it is obvious that you cannot stay at a 5-star hotel for 500 Turkish lira a night — a price like that is clearly too good to be true, and the law treats it as contrary to the ordinary course of life. But if, say, the hotel's normal nightly rate is 10,000 lira and you saw 7,000 lira, that is a believable discount, and you can insist on staying at the price you booked.