Google Authentication Api key not found

My website is based on PHP (Wordpress) and has been working perfectly fine until (29 September 2018) when it started to throw an error.

API Key not found. Please pass a valid API key

Here is the full stacktrace

Fatal error: Uncaught Google_ServiceException: 
Error calling GET https://www.googleapis.com/oauth2v2userinfo?key=DummyKeyForStackOverflow: (400)
API Key not found. Please pass a valid API key. in /home2/domainName/***/src/io/Google_REST.php:66

Stack trace:
#0 /home2/domainName/***/src/io/Google_REST.php(36): Google_REST::decodeHttpResponse(Object(Google_HttpRequest))
#1 /home2/domainName/***/src/service/Google_ServiceResource.php(177):Google_REST::execute(Object(Google_HttpRequest)) 
#2 /home2/domainName/***/src/contrib/Google_Oauth2Service.php(37):Google_ServiceResource->__call('get', Array)
#3 /home2/domainName/***/3rdparty.php(176):Google_UserinfoServiceResource->get()
#4 /home2/domainName/***/user_dashboard_profile.php(110): mymethod_goo in /home2/domainName/***/src/io/Google_REST.php on line 66

I thought probably the Api key is expired and regenerated and used it but the error is not fixed.

I didn't make any change to my website for last few months, so there are no chances of breaking a working code. Is there any recent update to Google API which I need to incorporate in my code.

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