What happens with your ticket if you are denied boarding due to a temperature check? (EU)

I know you didn't ask for this, but I'm going to do a frame challenge here and address the elephant in the room.

The Bulgarian government has posted multiple websites explaining the rules and regulations surrounding COVID in Bulgaria. These can be found at https://coronavirus.bg/ and https://www.mh.government.bg/bg/informaciya-za-grazhdani/informaciya-otnosno-noviya-koronavirus-2019-ncov/. While the latter site puts most of their information into Cyrillic-only PDF files with no Google Translate support (and as such doesn't allow me to quickly browse and search it), the former site does not explicitly ban people from moving with a fever (personal note: probably because it should be obvious to not travel with COVID symptoms).

Sofia Airport, on the other hand, explicitly mentions as their first bullet point:

Do not go to the airport if you have the following symptoms: fever, cough, difficulty breathing, loss of taste or smell;

Burgas Airport and Varna Airport:

Do not travel if you think you are ill!

Plovdiv airport:

Complete the Notification of Health status from your airline Don’t travel to the airport if you have been in any of the situation specified in the Notification of Health status

I cannot easily find this notification of health online, but it's likely that it mentions having a fever as one of the situations specified.

In case it's not obvious yet from these documents: most airports mention to not come to the airport if you are ill or have COVID symptoms.

Many airlines allows rebooking or refund of a ticked. They did this, to encourage passengers to book a flight on such period (where we never know about next lock-downs).

Note: airline websites have often the procedures on how to change/get refund in a prominent place, contrary to generalist booking website. Try there to get there information. In any case you should contact airline. Maybe try to avoid telling them about fever.

But you should check also what the local health authorities tell you about having fever. It may be illegal to go to airport with any kind of possible symptoms of CoVid, and this may be more expensive then the ticket.