How to fix Fatal error: Cannot declare class WP_REST_Blocks_Controller gutenberg plugin when upgrading to 5.X

This appears to be more of a general WordPress issue than a BoldGrid specific issue, but I’d advise first checking your plugins through the “wp-content” directory (since you’re unable to get into your site’s Dashboard). You will need access the source files of your site using FTP or through your host control panel. Delete or rename the directory for any plugins you’re not using. I’d advise contacting your hosting support if you have any trouble.