The first ticket of the day sent me to Microsoft SQL Server Management Studio to fix the sudden lack of database diagram support from a few of our databases.
“Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.”
The information provided by SSMS was very helpful but for those left unsure, the database has lost its owner and needs to be corrected, simply done with:
ALTER AUTHORIZATION ON DATABASE::databasename TO sa (sa can be changed to your preferred user)
Hope it helps! @dpitt