I recently updated to Android Studio 2.3, and now when I try to run the application, the emulator does not come online. It times out after 300 seconds.
Like urupvog's answer, make sure that you aren't running any other virtual machines like VirtualBox. When I restarted my computer, AVD worked until I started Vagrant for backend development (then it wouldn't launch).
See Android emulator and virtualbox cannot run at same time for more info.