1. Unplug, remove the Kali Linux install DVD or stick and retry.
2. Restore your HDD from your last backup. Only the very new don't backup prior to a major change like this. In which case you boot your W8.1 media and try Microsoft's automatic repairs (rarely works) and if no backups you decide what to do next such as pop in a new HDD to install W8.1 then add the old drive to get access to your files in hopes the OS install didn't wipe them out.