Can't connect to SQL Server DB from SSMS network error
As you said that the SQL Server Error code : 18456 is coming with state: 8. 

Mostly the Error code : 18456 with state : 8 coming due to user authentication.

As per MSDN Blog here the key to the message is the 'State' which the server will accurately set to reflect the source of the problem.

For Troubleshooting Error 18456 

As SQLBlog of @AaronBertrand here ,  you can find out every error details with description , based on their state with Error code: 18456.

After Edit

As SQLSERVERLEARNER Blog here  the


  Error Number: 18456 Severity: 14 State: 1


may have so many reasons.

Reasons for this error: 


User does not have access on the server 
Provided user name is wrong
Provided Password is Wrong
Using windows login credentials for SQL Server authentication
Login for the user is disabled
Login was successful but access to the server failed
Problems with SQL Service
Password for the user has to be changed 
User does not have access to perform the specified acton on the
server, Only has limited access
User does not have previlages to access the specific database,
 agent service etc
Even here MSDN BOL here
 mention that for State : 1 , Error information is not available.
 This state usually means you do not have permission to receive the
 error details. Contact your SQL Server administrator for more
 information.
As here
 State 1 now occurs when a login is disabled – but actually, the
 error in the log is 18470, not 18456 – because the login is
 disabled, it doesn't get that far.  See state 7. Prior to SQL
 Server 2005, State 1 always appeared in the log for all login
 failures, making for fun troubleshooting. The login is disabled
 and the password is incorrect.  This shows that password validation occurs first, since if the password is correct and the
 login is disabled, you get error 18470 (see state 1 above). It's
 possible that your application is sending cached credentials and
 the password has been changed or reset in the meantime – you may
 try logging out and logging back in to refresh these credentials.


For further your ref here and here