They told what happened
Matthew 28:8, “And they departed quickly from the tomb with fear and great joy and ran to report it to His disciples.”
Luke 24:9, “and returned from the tomb and reported all these things to the eleven and to all the rest.”
John 20:18, “Mary Magdalene came, announcing to the disciples, “I have seen the Lord,” and that He had said these things to her.”
They said nothing
Mark 16:8, “And they went out and fled from the tomb, for trembling and astonishment had gripped them; and they said nothing to anyone, for they were afraid.”
The best explanation is that the women initially said nothing (Mark 16:8) and then later told the disciples what they saw (Matthew 28:8; Luke 24:9). It would make sense that they were frightened and didn’t know what to do or say. But then later, of course, they spoke up. The John 20:18 account is chronologically later than the other references and is not relevant in this difficulty.
