One thing I did not know before is that a project in the iMovie uses the materials from the iPhone, instead of making hard copies. I deleted a lot of the original video clips from the photo library to save space, then when I opened the project in iMovie, it said videos are being downloaded from icloud, but it stuck there. This means the videos are no longer available.
The end of the world?
Fortunately, I used the Image Capture before to export the videos of iPhone photo library to my Macbook. Therefore, I can use the following to recover these iMovie projects.