Error message when you try to log on to Microsoft Dynamics CRM: “Cannot open database ‘[Organization]_MSCRM’ requested by the login” after restart server

After restart the server where I installed CRM2016, I have this error

Cannot open database “Organization_MSCRM” requested by the login.
The login failed. Login failed for user ‘NT AUTHORITY\NETWORK SERVICE’.

In the events viewer I watched the details:

Cannot open database “MSCRM_CONFIG” requested by the login. The login failed.
Login failed for user ‘Domain\CRMServer$‘.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: Cannot open database “MSCRM_CONFIG” requested by the login. The login failed. Login failed for user ‘Domain\CRMServer$‘.

This problem occurs if one or more of the following conditions are true:

• You install Microsoft Dynamics CRM in a multiple-server environment. Then, you install Microsoft Dynamics CRM directly on a server that is running Microsoft SQL Server. However, some SQL Server permissions are not set.
•   The Microsoft Dynamics CRM server is not added to the SQLAccessGroup group in the Active Directory directory service

I open the database and check the membership for the user NT AUTHORITY\NETWORK SERVICE on database MSCRM_CONFIG and the Database role membership db_owner was lost!!! I don’t know why!!!!

To resolve this problem I insert again the db_owner inside Database role membership and I reset IIS….

I resolve the problem!!!