Sometimes, when you try to start a debug session in Flex Builder, the tool will hang for a long time for no apparent reason. What is actually happening is that the connection between the IDE and the debug process is having trouble getting started, and the system is waiting for the connection to timeout.
You can shortcut this process by explicitly killing the hung process and restarting the debug session.
Step 1: Check to see if you have a task which is stuck by looking in the bottom right corner of your IDE window. There should be a small progress bar with a weird icon next to it. Click on the weird icon to open the progress view.
Step 2: Click on the stop button for the task that is stuck.