You need to check in your SQL Server Configuration Manager what services and instances you have installed. Go to Start Menu > All Programs > Microsoft SQL Server (version) > Configuration Tools and launch the appropriate Configuration Manager. You should see a screen something like this:

Find the SQL Server (.....) service(s), and make sure the service is started.
If you find a SQL Server (MSSQLSERVER) service started, then you have a (unnamed) default instance to which you can connect using
.
(local)
Arash-PC
You MUST NOT supply the MSSQLSERVER as the instance name!
If you find any other SQL Server (instance name) service started (e.g. SQL Server (SQLEXPRESS) in my screenshot), then the instance name (SQLEXPRESS) needs to be appended to the server name to connect to that instance:
.\SQLEXPRESS
(local)\SQLEXPRESS
Arash-PC\SQLEXPRESS
telnet localhost 1433- do you have a response (blank screen)? - Anton Krouglov