After a couple of wrong passwords, the sign-in page adds a quick "confirm you're human" check. Tick it and sign in as normal.
You will not see it when your password is right, and one mistyped password is not enough to bring it up. It appears from the third attempt, and it goes away once you have signed in successfully.
It is there because a wrong password can also come from a program guessing thousands of them. The check costs a person one click and costs a guessing program a great deal more.
Two other things guard the same page. Too many attempts in a few minutes are slowed down, and five failed attempts lock the account for fifteen minutes — you do not need to contact us, it unlocks on its own. If you are locked out and did not cause it, change your password once you are back in, and turn on two-factor authentication.