Create a Logs folder in the root of your website add this little snippy snappy snoo to your Global.asax file. This will catch all exceptions in your Asp.Net site and create a log file for you.
I'm not claiming credit for this code snippet, so if your reading this and discover I've copied some code from your blog, then you're probably right. The problem is I'm trying to keep little code snippets safe so I can refer back to them (hence, me adding this to my blog notepad) but I can't find the original source. If you are the source of this C# code then send me your link to original post and I'll credit you.