Alerted by Google and host of phishing pages that doesn't exist

Hello,

We received emails from both Google and our host telling us of a phishing pages that they claim appeared on our site. However, none of the pages appear anywhere in our list of Pages in our admin.

The pages are:
http://www.ironblockbuilding.com/from/dropbox

And…
.com/simple/dropbox/
.com/simple/dropbox/form.php
.com/simple/dropbox/index.php

Is this truly a breach of some sort? Does anyone have experience with this problem?

Thanks,
Bergie