Normally when you receive an error message with the
Error: 18456, ... it will contain additional information somewhere.
Detailed SSMS Login Error
In the case of the SSMS Login, the error message will be displayed together with a couple of icons. One of them is page with a white cross on red background. If you click on that icon you will be presented with the detailed error message, which could look a bit like this:
The detailed error message will contain additional information:
Server Name: .
Error Number: 18456
Line Number: 65536
Detailed Login Error in ERRORLOG
Additionally you can find the full error message in the ERRORLOG of your SQL Server instance. The entry will look similar to the following:
2018-08-10 15:37:56.10 Logon Error: 18456, Severity: 14, State: 5.
2018-08-10 15:37:56.10 Logon Login failed for user 'bigger'. Reason: Could not find a login matching the name provided. [CLIENT: <local machine>]
Decoding The State Number
With the state number you can then go and check Aaron Bertrand's Blog's article:
If you can edit additional information into your question, then we might be able to find the root cause together.