ASP.NET 访问不了了？

用localhost访问本地默认的IIS配置的网站。发现访问不了了以前可以访问的。
系统XPSP2+IIS+VS.net。
访问的时候提示是（友好提示是505内部错误：
Server Application Error
The server has encountered an error while loading an application during the processing of your request. Please refer to the event log for more detail information. Please contact the server administrator for assistance.应该从哪里下手？