Cannot locate or connect to SQL Server


I'm using BDE 5.1.1 with SQL Server 7 and I've been having problems
connecting to the server since yesterday. It happened without any
reasons, 'cause nothing has changed. I get the following error :

"Cannot locate or connect to SQL Server. Unable to connect. SQL Server
is unavailable or does not exist. Access denied."

Can someone help me with that ?

I have try re-installing BDE, deleting my objects declared in BDE and it
didn't do any difference.