忽然出现ERROR 503, 重启IIS也不行。查看C:\WINDOWS\system32\LogFiles\HTTPERR\httperr1.log，有ERROR LOG
503 AppOffline ASP.NET+v4.0
在IIS MANAGER里restart application pools > asp.net v4.0 and asp.net v4.0 classic, and then restart iis，竟然搞定了。
REF LINK: http://www.cnblogs.com/onlytiancai/archive/2007/06/03/769309.html