Can't connect to/from SQL Server 2008 from my Windows 2008 server
I'd check Services to ensure both are running - the name in Services for both SQL Servers will also confirm the instance name. On my laptop, I have MSSQLSERVER, so I can connect to that with either the machine name (LAPTOP), or with .    My other instance is called SQLTEST2, so I can connect to LAPTOP\SQLTEST2.

If that fails, go looking in both sets of binaries in Program Files. Each instance will have ERRORLOGs in the the respective log folders - that might have a clue in there, such as a login error.

Incidentally, since they are both on the same machine, ensure Shared Memory is enabled for both.