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 had a similar problem when updated my android studio. Somehow it changed my SDK path. I just changed it to my updated SDK path and it worked.
Android Studio -> File -> Settings -> Appearance & Behaviour -> System Settings -> Android SDK
Here you will find Android SDK location just click on edit link in front of that and browse and select the Android SDK from the file browser. Select and click on Apply.
Select AVD manager and start Emulator. This solution worked for me.