You need to check in with MU but this will only work if CZ has endorsed the ticket properly and given you a confirmed reservation on the new MU flight. If not, MU will probably send you to the CZ counter to have the problem fixed and this will probably mean that you miss the flight. Do you have any documentation for your changed ticket and reservation? Can you find your itinerary and ticket on the MU website? CheckMyTrip? Or have you tried (believe it or not) the Saudia website to search by name and ticket number?

My advice would be to try to call MU and ask them to verify that everything is OK in their system (ticket and reservation/itinerary: you need both of them to match) for you to fly their MU flight 2544. You should also ask them to give you the PNR in their system, which should probably be different from the CZ PNR. Take this information with you to help if the check in agent cannot find your reservation or ticket. I assume that OLCI is not an option.