If the settings page is empty and all you see are the Reset and Save buttons (and the boxes on the right), see question C29 below.
Start by making sure you are using the latest WordPress and theme versions.
If that is true, then this issue is usually caused by a poorly written written plugin that load their resources over the entire dashboard (instead of only where they are needed) and cause conflicts with scripts used by the theme.
If this method is complicated or the development tools show no errors (or point to files that cannot cause issues, like WordPress’ on script files), then the alternative is to disable all plugins and check if the settings page starts working / returns to working condition.
Then re-enable the used plugins one by one and re-check the theme’s settings page until the issue re-appears. That will indicate that the last enabled plugins is the issue.
The only solutions are to either contact the plugin author and nicely ask him/her to correct the behaviour and load plugin resources only where they are needed, stop using the plugin or disable it every time you need to use theme settings. There’s nothing we can do about this as a theme cannot affect plugins’ functionality.