Android Studio : emulator is already running

This worked for me:

Go to AVD Manager and click on the down-arrow on the right and click Stop. This stops the emulator and you can re-run it.

You don't have to completely delete the emulator when this happens. The files to delete should have a ".lock" extension:

You don't need to touch the configuration files.

You can easily find the files location by clicking on the arrow under the Actions tab (in the ADB manager), and selecting "Show on Disk".