The day my flex Debugger stopped working was the start of a very frustrating experience. I read forum after forum. To no avail I still couldn't get my Flex debugger to start working. I had a power failure a while back that completely wrecked Flex builder 2 to the point to where it wouldn't even open. I was forced to uninstall and reinstall Flex Builder 2. I got Flex Builder 2 working again but my debugger had completely stopped working. What would happen is my debugger would sit there and try and load and then all the sudden I would lose my internet connection and the debugger would stop running and display an error message that said check to see if flash debug version was installed. My first thought was oh okay no big deal I will just install the flash debugger again. You can find that here.After installing that I tried it again and still the same thing flex debug would never load.
I started reading forums. The first thing I tried was to write a very simple application to make sure it was in fact hitting the correct flash debugger version. My debug version and everything looked just fine. You can find the script I ran below if it does not come back with a debug version you will need to download it from the adobe web site here
I had trouble with "Launch in background" checkbox. When i uncheck it, Debugger running again!