To sign in to Matchday you need to ensure you have one of the required roles for access, which will most commonly be a team manager/coach, player/parent, or club treasurer/secretary.
You then need to enter your email address and connected password. This combination of details will be the same ones that allow you to sign in to other connected websites such as myaccount.thefa.com or englandfootball.com.
This sign in page allows you to reset your password using the 'forgotten password' button, or 'Sign in using FAN' if your email address is having issues.