Did you download the backup when you generated it? Was it a full backup or an incremental backup? The backup.zip.php is a temporary file containing temporary backup data and can only be read by the plugin. To get the backup file itself you need to have downloaded it using the plugin when the backup finished, or if it was an online backup it will be stored on our servers.
You might be able to try and see what temporary data is still in that file but it won't be simple. You'll need to strip a specific number of bytes from the beginning of the file so the file starts with the letters PK and it should become a ZIP file - but as to whether the contents are complete or even readable I don't know as it varies what sits in the file - this is all assuming the backup had completed perfectly and was awaiting download but never was downloaded. The exact number of bytes varies from platform to platform so I wouldn't be able to say.