While editing video with iMovie, suddenly some video clips and photos/stills got blurry.

While editing video on my late-2017 iMac (running OS Ventura 13.6.7) with iMovie (v. 10.4), I left the computer while it was working on editing tasks, and it put itself to sleep later. When I came back the next day to continue editing I noticed that two things had happened: 1) Some video clips and photos/stills got blurry in the timeline though they look fine in the media gallery and in the Photos app; 2) Many of the photos are now "missing remote media." I had heavily edited the still photos, so I hope I can make them reappear rather than starting over editing them. What went wrong and how can I fix these two problems. Thanks!