I have had several ocassions when I debug an application from Flash Builder and the application loads but it fails to connect to the debugger. If your Flex/Flash builder stopped working all of a sudden and if it’s stuck loading with the following message: “Launching: Waiting to connect to the running application…” there might be a few things that you could do.

1> Close Flex/Flash Builder and restart.
2> Clean the project and restart Flex/Flash Builder.
3> Get the latest version of Flash player debugger from http://www.adobe.com/support/flashplayer/downloads.html and try again.

If all this fails then this weird sort of solution worked for me.

Right click on the Flash/Flex application, select Debugger.
Select the option “Other Machine” for the question “Where is the debugger or host application running?” and enter the IP address as 127.0.0.1 and click Connect. This has resolved the issue for me. It’s kind of strange that I needed to switch from localhost to 127.0.0.1 to make it work and if anyone knows the reason why then please leave a comment here.