Suddently, my website is down and get Error "http error 503. the service is unavailable." I look up in Logs and get the message "Sequence contains no matching element"
Searching in interner I found some thread solved this problem at here
Following these steps I solved my problem:
1. Connect SQL Server and run the query. Using generate script in SQL Server to script table Webserver. Save sript to your local drive
2. Select * from Webserver . You will see the duplicate row
3. Using Truncate table Webserver
4. Open the script that generated table Webserver and run it
5. Finally, Recycle your IIS