From the Monarch CAA website page:
However, those who booked flight only after 14 December 2016 and are still in resort but are no longer covered by ATOL will have to pay for any additional costs such as extra accommodation if their flight does not take place on their original return date.Will I be charged for my flight back to the UK? If you are due to fly back to the UK on Monarch Airlines on or before 15 October 2017, or if your travel after that date is ATOL protected, you will be brought back to the UK at no cost to you. The Government will be seeking to recover some of the costs from debit and credit card companies in due course. In order to help with this, you may be asked to provide some information about how you paid for your original booking. You will not be charged for the new flight.
Passengers with Atol protection are entitled to reasonable accommodation and subsistence costs if they are delayed by more than four hours.
Those without Atol protection may be able to claim from credit card providers or insurers. Further information will be made available via the website. The CAA will process your refund as quickly as possible - check the dedicated website for more information.