I tried login with the master and tempdb databases and alter the login using the below command:

Use master
GO

ALTER LOGIN [MonitorDB] WITH DEFAULT_DATABASE = TempDB
GO

Any suggestion. Thanks

Have you tried using the GUI?  Right click the login.  Click on Properties and update the database default?

Could you describe you problem in detail? There is not much information in your post. Also, I can't find any error 4046 in sys.messages. Please include the full error message you get - and what you are trying to do when you get it.

ALTER LOGIN [MonitorDB] WITH DEFAULT_DATABASE = master
GO

check and confirm.