So here are the causes of this issue:
1- Moving your assets to a different folder.
2- Renaming your assets to a different name.
3- Changing the JSON file to a different place.
4- Changing the extension of your assets (For example from JPEG to PNG)
5- Deleting your assets.
You need to keep track of your assets at all times, make a main folder that contains everything and make everything clean as possible so you don’t get lost or confused when the assets starts to pile up, always work in a clean environment especially with big projects.
I also made a full post about this here a while ago (It’s not the best or the cleanest way) but it’s a start for sure: The art of your project folder
Hope that helps : )