I faced this problem when I was working with the Android Eclipse plugin and sometimes my application or the application changes would not show up on the emulator.
You might want to check out this link which Google has for the same problem. However in my case it was not helpful.
What worked in my case was killing the adb process. If you are using Ubuntu then goto System->Administration->System Monitor and hunt for the adb process, which would be sleeping. Kill the process and rerun your application.
On Windows you can go to the task manager and kill the adb process.