If you've booked a business plus fare with Ryanair you can change it up to an hour after the flight departed for no change fee, but any price change is payable. You can't do that if you've booked a standard fare.
With Easyjet, if you buy Missed Flight Cover and arrive up to 4 hours after the flight departs they'll put you on the next flight with seats, but not if you don't take out that option.
Monarch don't have anything similar. Their website makes that clear.
"It is passengers’ responsibility to arrive at the airport with plenty of time to allow the necessary check in and security procedures.
Failure to check in on time may result in you missing your flight, in which case Monarch cannot accept responsibility for making alternative travel arrangements. Your time will become invalid and no date/name changes will be allowed.
If you have missed the flight and are still at the airport, please visit the Monarch’s ticket desk, who will check the next flight available and its cost.
Alternatively, you can visit Monarch.co.uk and purchase a new flight.
If you booked your flight via a travel agent or tour operator, please contact them directly."