uhhhh...this is a very broad error. What this normally means is that the aspnet_wp.exe process has died/is not running on your server. (aspnet_wp is the process that handles .net execution)
This can happen for about 10^54 reasons. The process consumes to much server memory, the request takes to long to complete, etc. I can't help you beyond that without more information.
"The one language all programmers understand is profanity."