SQL Error - On Sharepoint Server

Hello

I have Sharepoint databases on Sharepoint server. And I checked Event Viewer on the server. And I get following error message continuously.

"An exception occurred while enqueueing a message in the target queue. Error: 15517, State: 1. Cannot execute as the database principal because the principal "dbo" does not exist, this type of principal cannot be impersonated, or you do not have permission."

I have no idea what is this error. And how i can solve this?

Please help

Thanks