I don't know my Adobe ID

Hi, 

Adobe Digital Editions is no longer letting me transfer books to my e-reader - it says they don't have the right permissions. This was when I clicked the option to unauthorise my computer, and I think I need to authorise it to be able to transfer files? However then it asks me to sign in with my Adobe ID number, which I don't know/have forgotten. How do I find this, or reset it? 

When I go to 'create ID' it tells me an account aleady exists with my email address, so I must have done it in the past. 

Thanks for any help.