Login failed with sa user or system administrator

To fix this:

1. Go to "SQL Server Configuration Manager"\ SQL Server Services.
2. Then right click the service of the SQL Server instance that is not accessible, and choose properties.
3. open the "Startup Parameters" tab.
4. There, insert "-m" in the "Specify a startup parameter", then click the Add button
   
   
   
5. Click OK
6. Restart the service.... after restarting, the SQL Server instance will enter in a single mode.
7. Now login to that SQL Server instance using a system admin.
8. change the password of the sa user or add any user you want with whatever permissions you want.
9. After you are done, go back to SQL Server service properties window, and again to the Startup Parameters tab, then remove the "-m".
10. Click OK, restart the service, and that's it.