Do you remember, would you have used a compare site to buy your insurance? If so you might be able to recall previous quotes when you log in to the site again. It might jog your memory as to which one you bought. The. You could phone them for a replacement certificate.
I sympathise, its easy to get email confirmation of something and not to bother to file it or print it off. If my life depended on it I wouldn't know who I last insured my car with. I would have to hope I filed it. off to check!