Many found that their accounts had been effectively reset, as they were greeted by the message Google welcomes new users with.
Their frustration swiftly found its way onto online message boards where they complained that everything had been lost.
"I have lost all on my emails/folders etc from Gmail. Why would this happen? How can I restore everything?" wrote one frustrated user.
"All my account settings, email, labels, contacts etc has just disappeared," said another.
"It is clear from the Gmail forums and Twitter that hundreds if not thousands of people have had their Gmail accounts compromised. A firm believer in the concept of cloud computing, it never occurred to me that my Gmail account could one day disappear," one user, LibertyGirlLondon, wrote in a blog post.
Meanwhile, Google is spending several frantic hours trying to restore the service, used by an estimated 200million people.
'We've already fixed the problem for some individuals. Our engineers are working as quickly as possible and we hope to have everything back to normal soon. We're very sorry for the inconvenience to our customers," a Google spokesman told The Huffington Post on Monday.
Initial estimates were that 0.23 percent of all users were affected, but Google soon revised it down to 0.08 percent, or around 150,000 users.