Hi @[deleted] Jewett,
Please be aware that in order to open the project on your computer that was created in PhotoStage, all elements that you used in your project (images/videos/audio files) need to be in the same folder as the project. If one of the elements (video/audio/image) is missing, the project will not open because it can not read the one file that is missing.
In order to avoid this situation in the future, when you finish you project click Menu->File->Back up Project File to Folder. This will create a folder on your computer that will include all elements (videos/audio files/images) that you used for the project and the project itself.
The other method would be to simply Export your file and you can later play it with your player.