Solution One:
Something, this problem can be network related, -> check if your network is behind a proxy. If so, you need to configure proxy on IDE Eclipse. For that, go to Windows -> Preferences -> General -> Network Connections and fill in your proxy info.
Solution two:
Clean up your project-specific meta data directories (which are stored under your workspace directory)
Solution Three:
Check if an adb process is running. If so, kill the adb process -> and restart Eclipse
Good luck :)