Matthew 9:18

18 While he spake these things unto them, behold, there came a certain ruler, and worshipped him, saying, My daughter is even now dead: but come and lay thy hand upon her, and she shall live.

Mark 5:23

22 And, behold, there cometh one of the rulers of the synagogue, Jairus by name; and when he saw him, he fell at his feet,
23 And besought him greatly, saying, My little daughter lieth at the point of death: I pray thee, come and lay thy hands on her, that she may be healed; and she shall live.

The account in Matthew says that Jairus's daughter was already dead, but Mark says that she was at the point of death.

I watched Bart Ehrman raise this contradiction, and of course, he dishonestly didn't mention the account in Luke, which he no doubt knew existed, and is the key to understanding the story:

Luke 8:41-50

41 And, behold, there came a man named Jairus, and he was a ruler of the synagogue: and he fell down at Jesus's feet, and besought him that he would come into his house:
42 For he had one only daughter, about twelve years of age, and she lay a dying. But as he went the people thronged him.
43 And a woman having an issue of blood twelve years, which had spent all her living upon physicians, neither could be healed of any,
44 Came behind him, and touched the border of his garment: and immediately her issue of blood stanched.
45 And Jesus said, Who touched me? When all denied, Peter and they that were with him said, Master, the multitude throng thee and press thee, and sayest thou, Who touched me?
46 And Jesus said, Somebody hath touched me: for I perceive that virtue is gone out of me.
47 And when the woman saw that she was not hid, she came trembling, and falling down before him, she declared unto him before all the people for what cause she had touched him, and how she was healed immediately.
48 And he said unto her, Daughter, be of good comfort: thy faith hath made thee whole; go in peace.
49 While he yet spake, there cometh one from the ruler of the synagogue's house, saying to him, Thy daughter is dead; trouble not the Master.
50 But when Jesus heard it, he answered him, saying, Fear not: believe only, and she shall be made whole.

In light of the above, we can piece together what happened:

• There is commotion as people throng Jesus. Jairus comes to Him and tells Him that his daughter is near death
• There is a distraction where a woman grabs Jesus's garment. During this distraction, one of Jairus's servants comes to Jairus and tells him that his daughter died
• Jairus then tells this to Jesus

Conclusion

Once again, the Gospels do not contradict each other. They supplement each other, and we can compare them to get a more complete story of what took place.