If you cannot log into your account anymore and used to have a valid Riseup account, chances are your account has been deactivated. After a certain amount of time, inactive accounts are disabled. Eventually, if it is not reactivated, the account is deleted. Deleted account names are kept in a reserved names list and won't be available for new accounts for several years.
To reactivate your account, you can log in at https://account.riseup.net. If you can log in successfully, your account has been reactivated. If not, chances are it has been deleted. If that is the case, sadly there is nothing we can do for you.