Hi,
The first thing to check is what changed in your environment at the time that the fault started to occur. For example:
- were service packs or hot fixes applied at any part of the environment: SharePoint, SQL Server, Windows Server.
- network configuration changes (routers, firewalls)
It may well be that some reconfiguration has caused the probelm. You should also check the event logs on all servers (web and SQL), as well as the SharePoint logs contained within the Diagnostic section of the Central Administration site.
Without more detail around these items, it would be difficult to say what could have started the problem.
Cheers,
Andy