British tourists, flying with Thomas Cook, queue at the Enfidha International Airport in Tunisia September 23, 2019. FETHI BELAID/AFP/Getty Images

Around 600,000 travelers are affected by the collapse of tour operator Thomas Cook.

The countries where the largest numbers of tourists are stranded are Spain, Turkey, Tunisia and Greece.

Here's what affected travelers need to know:

I'm currently on a Thomas Cook holiday. How do I get home?

The Civil Aviation Authority (CAA) will arrange return flights for ATOL-protected travelers currently abroad who were planning to fly back to the UK with Thomas Cook.

What is ATOL? It stands for "Air Travel Organiser's Licence" -- it's a UK financial protection scheme that safeguards most package holidays sold by travel businesses based in the UK.

Basically, it prevents travelers from being left without assistance or facing a financial loss if their travel company stops trading. Which is exactly what happened in the UK on Sunday.

For those currently abroad and due to fly back to the UK with Thomas Cook, the CAA is providing new flights to get them home.

What about hotels?

Now for the bad news: Travelers who only booked hotels through Thomas Cook and not flight-inclusive packages will have to foot the hotel bill themselves.

But if your Thomas Cook holiday included flights, there is specific advice about accommodation.

The CAA says that if you are currently abroad on an ATOL-protected package holiday with Thomas Cook -- or any company which is part of the Thomas Cook Group -- the authority "will seek to guarantee your stay directly with your hotel."

The CAA says it may take a few days to secure these arrangements. In the meantime, do not make a payment to your hotel, unless instructed otherwise by the CAA team.

If you are not ATOL-protected, the CAA says you can't make a claim for out-of-pocket expenses and additional nights of hotel accommodation -- but you may be able to claim from your travel insurer, bank or credit card issuer.

I haven't traveled yet. How do I get a refund?

Don't bother going to the airport. All Thomas Cook flights were immediately canceled following the announcement of the company's shutdown.

As for getting a refund, again, in the UK it all comes down to ATOL. If your holiday was protected, you should be able to file a claim via the ATOL scheme.

Now here's where things get a bit tricky.

The CAA says if you booked a holiday with Thomas Cook but your flights are with an airline unrelated to the Thomas Cook Group, your flights may still be available.

"You are advised to check with the airline if you are still able to travel," says the authority's advice.

"If you wish to travel, you can do so and make a claim to ATOL for the cost of replacing your other travel services. You are not obliged to use your flights and can instead claim a refund under the ATOL scheme."