Hello,
Please check the Risky Users list in Azure AD for the user's account (and verify it hasn't been compromised):
Security | Risky users
https://aad.portal.azure.com/#view/Microsoft_AAD_IAM/SecurityMenuBlade/~/RiskyUsers
It is possible that you have a Sign In Policy that blocks users when they get a certain risk level:
Identity Protection | Sign-in risk policy
https://aad.portal.azure.com/#view/Microsoft_AAD_IAM/IdentityProtectionMenuBlade/~/SignInPolicy
If this is helpful please accept answer.