I have the exact same question as the person here Black screen and no audio but I don’t understand the solutions with proxies etc
After a bit more of searching, I found the problem, I moved the source material. After I moved it back into the folder where it was when I began the project, everything was fine.
Please remember, Shotcut does not make a copy of the source images and videos you specify. It just keeps a note of the filenames. So if after working on a Shotcut project you delete the files, or rename them or put them in a different folder Shotcut has no knowledge of where the video or image data then resides.