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.
Additionally,
I too had the same problem, then I went to AVD manager and right click on the emulator and stopped it and I RUN the application again and this time it worked. it may be a temporary solution but works for the time being.
AVD manager-> right-click on the emulator you are using -> Stop
Now Run
your application again.
Note: Sometimes closing the emulator directly is not working for the above-mentioned problem but stopping it from the AVD manager as mentioned is working.