It seems like you had separated the contact management database before. This process automatically creates a new connection string "CMSOMConnectionString" in a web.config file pointing to the newly created DB.
However, it seems like the connection string is not specified properly. Could you please check if its value is set correctly, or if you have the
<add name="CMSOMConnectionString" connectionString="..." />
key in your web.config file?
Please also check if accidentally configured the DB to use 2 DBs - the original one and the separated one in Site Manager -> Administration -> System -> DB separation? If the button displays message "Join contact management DB with main DB" as in the attached screenshot, please do so.
Best regards, Martin