You can get this error due to various reasons. Can you please make sure that -
- The work or school account that was used in the configuration wizard to set up directory synchronization is not deleted or disabled and the account password is not expired.
- The logon account for one or more directory synchronization services is not deleted or disabled and account password is not expired.
- Verify if admin account used for directory synchronization is not changed.
Can you also check if Microsoft Azure AD Sync/ is running properly.
If yes, then get the logon account on Azure AD Sync by right clicking and checking the Log On tab on Properties window.
Try to find the logon account on a domain controller by going to Active Directory Users and Computers (Dsa.msc), right clicking the Domain Name and Find the logon account name. If you do not see the account name then try to find it on Active Directory Recycle Bin in case it got deleted.