Error after removing sql server 2008 R2 Express edition and instaling Entreprise edition: Unable to connect to MyHost\MSSQLSERVER
For a default instance (MSSQLSERVER), you only specify the computer name when connecting; the instance name (which is actually a ficticious placeholder) is implied.

Also, if you're connecting from another machine now, you may need to configure the Windows Firewall to allow connections through. The error message you get will be the same as the one you're seeing now. See my video here for more on that.