“There's been a change to your Google account” message and password prompt?

It looks like this is a known issue, and Google is on top of it.

From the product forums:

We've gotten reports about some users being signed out of their accounts unexpectedly. We're investigating, but not to worry: there is no indication that this is connected to any phishing or account security threats.

Please try to sign-in again at accounts.google.com and if you cannot remember your password, please use this link (g.co/recover) to recover your password. If you use 2-Step Verification, there may have been a delay in receiving your SMS code. Please try again or use backup codes.

Essentially the same message is currently displayed at the top of the Accounts Help: Can't sign in to your Google Account page.

Also, from @google on Twitter:

We know some of you had issues signing in today. Please try again now. Rest easy -- your account's security was not affected.

See also:

• Information Security Stack Exchange: Google account verification request


• Google accounts are on the fritz but nobody is getting hacked


• PSA: If you were signed out of your Google account tonight, don’t worry


• Google is having issues with account sign-in, resulting in errors with Android devices, OnHub, and Wifi


• Google Users Unexpectedly Signed Out of Accounts