As Alpesh said, you must finish adb.exe.
Personally, I like to open Command Prompt and type the following lines:
> adb kill-server
It finishes adb.exe process. It should return the message:
* server not running *
Then I restart the process:
> adb start-server
It restarts adb.exe. It should return the message:
* daemon not running. starting it now*
* daemon started successfully *
Then, try to run your project again. At least for me, this problem happened not so often after I increase my memory capacity (from 512MB to 1GB), but I don't know exactly if this problem has relation with the pc memory.
Well, if after using kill-server and start-server the problem persists, then close the emulator and start it again. If the problem is still happening, unfortunatelly you have to restart your pc.