The only possible methods to produce a password reset is having an alternative e-mail set up (see https://account.riseup.net/reset_password_token/new) or having generated and stored a recovery code (see https://account.riseup.net/reset_password).
This recovery code would have normally been created and presented to you when your account was created. If you did not retain this, you can create a new one (see https://account.riseup.net/passwords?view=recovery), assuming you have your password.
Resetting your password using the email recovery method will result in destroying all of your messages. This may seem a bit extreme, but we do this to protect our users and their information.
If you have not forgotten your password but you still cannot log in to your account and you haven't logged in for a while maybe your account has been disabled.