Lots of places to start checking, but try the SQL Server Error logs first. Look for the following entries:
If something is amiss here, google the error message and start from there.
2006-01-26 11:09:34.59 server Using 'SSNETLIB.DLL' version '8.0.2039'.
2006-01-26 11:09:34.62 server SQL server listening on xx.xx.xx.xx: 1433.
2006-01-26 11:09:34.62 server SQL server listening on 127.0.0.1: 1433.
2006-01-26 11:09:34.71 spid3 Server name is 'MYSERVER'.
2006-01-26 11:09:35.62 server SQL server listening on TCP, Shared Memory, Named Pipes.
2006-01-26 11:09:35.73 server SQL Server is ready for client connections
Next try telnetting to your server's ip address on port 1433 (or whatever port SQL says it's listening on). From a command window, type telnet MyServer 1433. The command window should blink, go entirely black and nothing else. If you get a response like 'cannot connect', then there are network issues (or the SQL Server service is not started.
Try starting there...
Have you hugged your backup today?