My emulator was working fine yesterday and suddenly with my other classmates it is not working anymore today. I am back at home with my home network which should not have any firewall restrictions but it is still not working. Is there any suggestions? Thank you.
This is the error we are getting.
même problème aujourd'hui. L'émulateur ne fonctionne plus !!!
It appears there is an issue at MIT or with the Chrome browser. I too have the identical issue using Chrome. No issue using Firefox. 
Until the issue is fixed, use Companion with a real device for live development. or use Firefox.
Firefox works with my system, Companion works but Chrome and the emulator does not. 
MIT has been informed there is an issue.
-- Steve
Thank you Steve. I have managed to get it working with Firefox!
Was spending the whole afternoon uninstalling and reinstalling. 
... it is due to a Google Security feature. Web Apps like App Inventor have to move off http and on to https, but a tremendous amount of work is required to change everything. MIT are granted a temporary exemption, which had to be renewed.
So, AI2 should again be fine on Chrome.