It sounds simple but lots of people use more than one email and knowing which one you've used for The FA's sites can be difficult.
Either head to the Whole Game System main page and attempt a login with what would most likely be your email. If you can successfully log in without receiving a duplicate email error message then you're fine to log in to Matchday with the email and password. If you do receive an error message then follow the instructions here.
You can also use the retrieve a FAN function to find out your email.
You can find contact details for any county FA by searching [county] FA, then the 'about' tab and 'contact'.
If you do not know your password: Head to https://www.thefa.com/account/RequestPasswordReset and follow the instructions. You will need to know either your FAN or your connected email to do this.