There are multiple reasons, why you can't download the results of your rendering.
Pulze Scene Manager
If you rendered your scene via Pulze, the following reasons could have caused the download issues.
No output path
There is no output path defined in Scene Manager and the render settings:
Resolution: Set the path where you like to save your result files.
Output path defined but not available
Your output path (e.g. your network storage) is defined in the scene, but is currently not available. Usually, this happens when users e.g. the local network storage is not available, or you switch from your office workstation to another PC where you have a different filesystem structure/paths.
Resolution: Use the download-to function or switch to the right machine.
Helio "direct" user
Can't download the results
If you render your scene via Helio, you might not set an output path, or you switched your machine, so the output path is not available on this machine.
Resolution: Set the right output path in the project settings.
Workaround: Use the "download-to" button to download the results quickly.
Pro tip: Filetransfer status
Go to the filestransfer (download/upload) feature and check the current download and potential issues: