If you cannot access the e-mail address associated with your Cloudflare account or can't discover which e-mail address it is, you would need to open a new user account with Cloudflare and add the site you wish to have control over to that new account. Adding the domain(s) to a new account will require going through the signup process to get the domain(s) active on Cloudflare again.
As part of the sign up, new Cloudflare name servers will be provided to you, and when you configure your domain to use these at the registrar level, the domain will automatically transfer into your new account (and be disabled from the previous one).