Unfortunately, it is not possible to backup an iPhone that won't turn on to iCloud. However, you can still try to backup your data in a few ways. First, you can try connecting the iPhone to a computer and using iTunes to back up the device. If that doesn't work, you can also try using a third-party recovery tool to extract data from the device. Finally, you can also try to contact Apple Support and see if they can help you recover any data from the device.