When attempting to change the system administrator (SA) password by changing the value in the Password
box of the SQL Server Login Properties dialog box (on the General
tab) in the SQL Server Enterprise Manager, you may receive the following error message:
Error 21776: [SQL-DMO] The name 'dbo' was not found in the Users collection. If the name is a qualified name, use  to separate various parts of the name, and try again.
This problem can be the result of performing a restore to a server where the login that was mapped to the database owner (DBO) on the source server does not exist on the destination server. In this case, one or more DBOs are mapped to NULL rather than to a specific login. The SQL Server Distributed Management Object (SQL-DMO) collection will have no record of a login for 'DBO' and will not allow the SA password change.
To work around this problem, use the sp_changedbowner stored procedure to change the owner of any databases owned by a login that does not exist on the new server to a login that does exist on the server.NOTE:
When this problem occurs, you can still use the sp_password stored procedure to change the SA password. However, this option is not recommended because it does nothing to resolve the situation of the missing DBOs.
For more information on the sp_changedbowner and sp_password stored procedures, please see the SQL Server Books Online.