You need to post the URL if the forum to get help with that but you may be able to fix it yourself as Nabble had an upgrade recently.
If you are "Founder" and you still use all the email addresses likely to have used for that user then you can send them an email (click on their avatar and choose the option to send an email). Hopefully, seeing which email address was used may help remind you of the password used.
Failing that you will need to provide evidence that you were "Founder" and deserve access to that account.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.