Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Your account is accessed using a cryptographic key that resides wherever your acme client decided to save it. The specific account is not relevant or important unless you have a rate limit exception, if you lost the key just make another account.
Knowing what you'd like to do can help us respond.
Your "account" is just a private key on the server that runs your ACME client.
You may be able to get that from whatever client you're using (does certbot show_account show it if you're using certbot maybe?), but Let's Encrypt is trying to not know which email address are tied to which accounts for privacy reasons. They have email newsletters that you can sign up any email address you want for, though.
That great to know, so we have an existing certificate for our domain, that was set up by an unknown previous employee. We now have a subdomain that I need a SSL for, do I create a new certificate and if so what steps should I take? In order for the existing certificate to work would a validation file be on our server or the website hosting server ? If I don't know where the subdomain site is hosted can we validate the subdomain by using the main site hosted server?
Most of that depends on how you plan to use the certificate and what is serving the website you're hosting, as automatic renewal is an important part of the equation.
The most common way of setting things up is each server is running an ACME client, which handles automatically requests and installing certificates for the web sites running on that server. So figuring out how to get certificates before you've figured out how you're hosting your web sites is looking at it kind of backwards: Usually if you first get your sites running under http, configuring them to also serve https is relatively straightforward.