F8 no longer working in cubase 11 pro

For 2 weeks, working with video, I could use F8 to open video window, play in it, and scrub. Suddenly 2 days ago, I load the same program, but nothing happens when I press F8. It’s been very frustrating. All programs w/o video work fine. The video imported plays properly with audio, though I only see the thumbnails. Any help would be appreciated. Nothing changed with my setup except for the days of the week.