Last weekend, the passwords of 93,000 Pokemon GO players were hacked and published on Reddit. A downloadable file was posted on the sub-Reddit /r/pokemongodev with the full list. It was quickly taken down, but the damage was pretty much already done.
Some users noticed right away when their passwords refused to work. A quick password reset fixed things, but that may not be true for all accounts. If your password for the Pokemon GO account is the same as your activation email account, then there could be a lot more damage. In that case, you should be quick about fixing your account and changing your passwords.