Problems related to: 1) Service, 2) Network, 3) Login
Is SQL Service running as a service?
a) Master DB Corrupted?
b) Model DB Corrupted?
c) TempDB path not found?
nslookup (name resolution) to SQL Server instance
Ping Server IP
Do you log in to the windows server where SQL Server is installed to eliminate the Network. (Here you log in with Shared Memory Protocol)
Is TCP/IP (or Named Pipes) Enabled in SQL Server
Should this login be entered in SQL Server?
a) Has it been locked out?
b) Are there permissions to connect?
If authentication is done with the Domain Controller, can SQL Server be connected to the Domain Controller?
CONSULT SQL SERVER LOG AND WINDOWS SYSTEM LOG FOR EVERYTHING.