WordPress recently made a major change in 5.8 version where widgets are displayed in block editor style instead of classic style. Many users started to face the problem with loading the widgets after updating WordPress version to 5.8. There is no option provided to switch back to classic style in WordPress by default. Even WordPress haven't made any official statement on the same but we should expect a fix soon.
Solution 1: Fix WordPress widget not showing error by using plugin
For now, users facing issue with loading of the widgets can bring back the old classic style by installing a plugin called Classic Widgets developed by WordPress team. After installation, activate the plugin and widgets will be loaded in classic style. At any point you can switch back to block editor style by disabling the plugin.
Solution 2: Fix WordPress widgets not showing up in dashboard error by updating WordPress theme
The theme creators have started to take a note of the widget error & other issues introduced in WordPress 5.8. Therefore, check if there is a new update available for your theme and change log which mentions the fix and updates. If there is no update pushed by the theme developers then try reaching them out through their support channel
Solution 3: Fix WordPress widget error by downgrading WordPress version
The problem does not get resolve by switching to default WordPress theme or deactivating activating WordPress plugins. But you can consider downgrading the WordPress version to 5.7.x. Checkout the post below on how to downgrade the WordPress version.
Hope one of the above solution works for you and widgets are visible again from admin dashboard. Happy Blogging!