Hey there! Let's tackle your problem step by step. First off, rolling back to a previous Ubuntu version isn't a straightforward one-click process, and it comes with some risks. Before jumping into downgrading, let's first troubleshoot the localhost access issue—since fixing that might save you from the hassle of a system rollback.

Troubleshooting Localhost Access #

• Check if your local service is running: If you're using a web server like Apache or Nginx, run systemctl status apache2 or systemctl status nginx to verify it's active. You can also check if the port is being listened to with ss -tulpn | grep :80 (replace 80 with your service's port if needed).
• Verify firewall settings: Ubuntu 24.04 uses UFW by default. Run sudo ufw status to see if incoming connections to your local service are allowed. You can temporarily disable UFW to test with sudo ufw disable—just remember to re-enable it later with sudo ufw enable.
• Check your hosts file: Open /etc/hosts with a text editor (e.g., sudo nano /etc/hosts) and make sure it has these two lines:

  127.0.0.1 localhost
  ::1 localhost
  

  If they're missing, add them and save the file.
• Test with a command-line tool: Bypass browser issues by running curl http://localhost—if you get a response, the problem is likely with your browser (clear cache, try incognito mode).

Downgrading Ubuntu 24.04 (Last Resort) #

Warning: Downgrading can cause dependency conflicts, system instability, or even prevent your system from booting. Backup all important data before proceeding.

• Use a system snapshot (if available): If you created a snapshot with tools like Timeshift before upgrading to 24.04, this is the safest way to roll back. Boot into the Timeshift recovery environment (or run it from a live USB) and restore your previous system state.
• Manual repository downgrade:
  1. Open your sources list with sudo nano /etc/apt/sources.list. Replace every instance of noble (Ubuntu 24.04's codename) with the codename of your previous version (e.g., jammy for 22.04, focal for 20.04).
  2. Update your package lists with sudo apt update.
  3. Run sudo apt dist-upgrade to downgrade packages. You'll be prompted to confirm changes—pay close attention to any dependency warnings.
  4. Reboot your system once the process completes.

Remember, downgrading is not officially supported by Ubuntu, so proceed with extreme caution. If the localhost issue gets fixed with the troubleshooting steps above, that's definitely the better path.

备注：内容来源于stack exchange，提问作者Lew Lowther