There can be more than one reason to get this error page.. It can be anything to the services which are not running, IIS failures etc. It is always recommended to review the logs in windows to find possible errors.
Check Event Viewer for error logs (In this case SharePoint_Config database cannot be opened)
Go to SQL Server Management Studio and find the SharePoint_Config database..
If SharePoint_Config database is in suspect mode.. and not accessible.. do the following
SharePoint_Config SET SINGLE_USER
WITH ROLLBACK IMMEDIATE
DBCC CheckDB (‘SharePoint_Config’, REPAIR_ALLOW_DATA_LOSS)
ALTER DATABASE SharePoint_Config SET MULTI_USER
DBCC CheckDB (‘SharePoint_Config’)
Things should go back to normal after this fix…
ERROR 02 – Application pool is not running