lost access to my Google calendar

This morning, I discovered that I can no longer add items to my Google calendar and that any new items added on my phone did not sync. I tried uninstalling the Google Provider addon and then tried to resinstall it. However, I'm getting "Bad Gateway" error for the addons site. Having read a couple of articles I then tried to add my Google calendar as a network calendar and Google accepted the request but Thunderbird reports that my "credentials were not accepted" - despite the fact the Google sent me an email saying that Thunderbird now had full access to my calendar. Do I need the "Provider" addon? If so, where do I get it and why did it stop working?