Many of players experiencing account reset seems to have multiple google accounts. That means Pokemon Go automatically referenced another account which you did not use to sign up Pokemon Go.
To deal with this error, what you should do is simple: switching your google account to the one you used to begin Pokemon Go.
As this tweet noted, your account and data (caught Pokemon, powered-up CP, PokeDex, and others) are not deleted. You do not have to delete the app as noted in the tweet above, but you can see your favorite Pokemon by doing the following process.
Like those 3 tweet postings above, some Pokemon Go players thought that they had their account deleted due to update. However, it is an error simply. You can get your account back.