Sometimes you can't communicate with your running emulator although its up and running. There is nothing listed under "Devices" section of the DDMS view in eclipse. The "Location controls" section under Emulator control is also disabled.
Why you need that ? For example, you want to have your location using GPS on your emulator. That can be achieved by simulating GPS coordinates using DDMS view > Emulator Control > Location Controls. There you can assign longitude and latitude.
Try listing your running processes on your computer. On Mac you can use "Activity monitor". There you can find a process named "adb" (Android debug bridge). When you find it, kill the process and watch your eclipse console output for following message:
[2012-07-09 19:16:16 - DeviceMonitor] Connection attempts: 1
After few attempts you should have your connection to the emulator back. There should be an emulator listed under devices sections automatically.