The most likely reason for this is not having an index.html in the root of the files you upload. If you use the ZapWorks CLI then that will give you a warning if you are missing this file.
If you upload a zip instead, then make sure there are no unwanted folders in the zip - for example if you select a folder containing your files to compress, it’s possible the zip file will also include that unwanted folder so index.html will not be at the root.
We’ll add some tasks to make some improvements here - we should be able to detect the “unwanted folder” case and cope with that automatically, and we can also add better warnings if you upload a zip without the index.html in the right place.