Although the main cause of the massive outage is unknown and Google has not yet responded to the queries, it is being speculated that the services have been down due to a server end problem.
The error code 500 which is showing throughout the sites is a general response from the server that something has gone wrong. The problem could have arised when resources were requested by the server, or it could be an issue with outdated or corrupted cache and cookies on Google's end.
Google services, including Gmail, YouTube, and Google search, were down for millions of users across the world. Downdetector confirmed the outage saying that the issues started at around 11:56 GMT, and are affecting users worldwide.
However, of those who reported issues with YouTube, 54% said they couldn’t access the website, 42% couldn’t watch videos and 3% were struggling to log-in.
And of those who reported problems with Gmail, 75% couldn’t log-in, 15% couldn’t access the website, and 8% were not receiving messages.
Downdetector showed over 9,000 reported cases from users having issues accessing YouTube. Similar outage was reported on Gmail and YouTube.