If you are unable to see your team or league, please make sure you are logged into the NFL.com account with which the team is associated. Users often will draft a team while signed in with one username/email address and later sign in with a second email address and rightly not be able to access the league.
If you have been signing in with your email address, try signing in with your username instead. If you've already been signing in with the username, try the email address.
Users without a password or access to the email account associated with an NFL.com account will not be able to access that NFL.com account. You may want to create a new NFL.com account with an email address for which you do have access.
In a custom fantasy league, the Commissioner can use the Edit Team Owners tool to remove an old account and send an invitation to the email address associated with a new account at the same time.