There is likely a javascript error which you can find by opening the javascript console in the developer tools of the browser.
If you need to restore access to your forum, does the admin label work and can you disable the extension ? Otherwise use composer remove <package> with the extension package name that was used to install it, this will remove the extension and hopefully make the forum work again.
If you have SSH access, please share your php flarum info output as this provides useful information.
