I downloaded Photoshop sometime ago. I am now trying to installing it on a new computer. I can't find the serial number and since it has been a few years, the number is not on My Adobe. What can I do?

You might have to search more thoroughly to find it, otherwise, you need to contact Adobe Support either by chat or via phone when you have serial number and activation issues.

Here is a link to a page with options to help make contact:

http://www.adobe.com/support/download-install/supportinfo/