If you are in your Organizer account and cannot find your Past SignUps, chances are you do not have your list of SignUps set to show Past SignUps.
On your Dashboard, the first section is My SignUps. You choose what list of SignUps you wish to see by clicking next to the Filter By:
Click on the area to the right of "Show" and choose to show "Past SignUps" or "All SignUps".
If you still cannot find your signup, consider (and check) any other accounts you may have created under different email addresses.
If you are still not successful, please contact us at support@signup.com and let us know the Name and Dates of the SignUp. If you have an invitation link in any prior email, please also provide that to help us help you.