On 12 Sha‘baan 647 A.H., the Christians set out from Damietta crossing the Nile southward to Cairo, and it was certain that they should pass by Mansoura as expected by King As-Saalih Ayyoob. On the middle night of Sha‘baan 647 H., As-Saalih Najm Ad-Deen Ayyoob, May Allaah have mercy upon him, died, while in Mansoura, sharing with his leaders in putting the plan to fortify the city, we ask Allaah, the Almighty, to forgive him, bestow mercy upon him, and give him the reward of the martyrs. According to Ibn Taghri Bardi, the author of An-Nujoom Az-Zaahirah Fee Mulook Misr Wal-Qaahirah (d874 A.H.), "If Sultan As-Saalih Najm Ad-Deen Ayyoob had no good deed other than his fortitude when meeting the enemy in Mansoura, even though he was suffering from such chronic diseases, and then his death while involved in Jihaad and the defense of the Muslims, it would be sufficient for him…how patient, persevering and excessively valorous he was!"

It was a dangerous calamity on the Muslims, not only for the loss of their leader, but also for the lack of his alternative and successor, especially at this critical time and under those difficult circumstances, with the seaport of Damietta being occupied, and the Crusader soldiers on the way.
The wife of Sultan As-Saalih Najm Ad-Deen behaved very wisely. His wife was Shajarat Ad-Durr, who had previously been a slave-girl of Armenian or Turkish origins, belonging to As-Saalih Najm Ad-Deen Ayyoob, who emancipated and then married her. She then was closer to the Mamluks.
What did Shajarat Ad-Durr do after the death of Sultan As-Saalih Ayyoob?

She concealed his death, and claimed that the physicians had prevented anyone from visiting him, and quickly sent to his son Toran Shah, who was ruling a city known as Kiva Fort (presently in Turkey), reporting to him about the death of his father, which required him to come to take over Egypt and Shaam. She agreed with the vizier of As-Saalih, Fakhr Ad-Deen Yoosuf, to administer the affairs of the state until Toran Shah would come. She further required both Aqtaay and Beebars to go on their preparations for the decisive battle in Mansoura. Thus, things became better after the death of King As-Saalih, and the expected trouble because of his sudden death did not take place. But under such difficult circumstances, and in spite of all the precautions taken by Shajarat Ad-Durr, the death of As-Saalih Ayyoob leaked to the people, and even to the Crusaders, which, in turn, enthused them even more, and lowered the spirits of the Egyptian army, which, anyway, remained firm in Mansoura.

Both Aqtaay and Beebars put an excellent plan to meet the French army in Mansoura, and showed it to Shajarat Ad-Durr, who was acting as the ruler until the arrival of Toran Shah. She sanctioned the plan, and the Egyptian army took its sites, and prepared for the meeting.

https://islamstory.com/en/artical/20775/death-king-as-saalih