Cannot connect to SQL Server express from SSMS

Please see:

• Solving Connectivity errors to SQL Server
• Troubleshoot connecting to the SQL Server Database Engine

Use . or (local) or localhost for server name if you installed the server as default instance.

Use .\sqlexpress or localhost\sqlexpress if you have SQL Express.

The server name syntax is

Servername\InstanceName

If the instance is default you use just Servername.
For SQL Express, instance name is sqlexpress by default.