I suddenly have a blank preview and can't scrub in the timeline

First, check whether your project is set to 59.94 fps (open Help/Settings).  If so, try changing to 60fps, then force-close and restart LumaFusion.  This may resolve your issue.  If not, you may also want to try lowering the preview quality (open Help/Settings and tap on Preferences).

Please contact us at support@luma-touch.com if neither of these recover you preview, or if you find that nearly any edit within a project loses the preview again and again.