Login failed for user - Error 18456 - Severity 14, State 38
State codes and their meaning.
1      'Account is locked out'
2      'User id is not valid'
3-4    'Undocumented'
5      'User id is not valid'
6      'Undocumented'
7      'The login being used is disabled'
8      'Incorrect password'
9      'Invalid password'
10     'Related to a SQL login being bound to Windows domain password policy enforcement.
        See KB925744.'
11-12  'Login valid but server access failed'
16     'Login valid, but not permissioned to use the target database'
18     'Password expired'
27     'Initial database could not be found'
38     'Login valid but database unavailable (or login not permissioned)'

More detailed information is available in Aaron Bertrand's blog.
To find the status code, you need to inspect the SQL Server Error Log after ensuring that failed logins are recorded in the error log.