(1)Event Logs
(i)Any new recurring errors in Event Logs
(ii)check the HTTPERR log for Connection Denied Reports (usuallyC:\windows\system32\logfiles)
(2)IIS Application Pools or Websites
Check for any stopped app pools or sites in IIS Manager.
(3)Windows Services
Are any of them stopped?
(4)Event Viwers
(5)SQL Server
(i)Are you getting security access issues with accessing SQL server
(ii)Check logs aren't full? (Consult with DB Admin,to resolve it.if you finds this cause)
(6)Web Front End
Are you out of disk space??? SharePoint can produce huge amount of logging info and that can can eat up your disk!
......do revert if you find any issues.