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. One way of finding out which email is to use the retrieve a FAN function.
You could also ask your club secretary to click on your name in the 'player registration' tab. They can then click on your name and bring up your contact details which will have all the details we have on record. If you do not have an email address on record (you will need one to get into Matchday then email in to your county FA telling them your name, date of birth and which email you would like added to your account.
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.