Check whether the service is running on the server. You can do this by going to services.msc and looking for the SQL Server service.
If it is running, make sure you are using the correct server name.
Are you trying to connect from the same box where the server is installed, or from a remote box? If you are connecting from a remote box, try on the same box to see if that works.
If it works from the server box, on the remote box, first see if you can ping the remote server. Also, if connection is working locally, but not from a remote box, verify that remote connections are enabled. You can do this in the server properties dialog in SSMS or run a script
EXEC sys.sp_configure N'remote access', N'1'
RECONFIGURE WITH OVERRIDE