The Problem:

Upgrading Hikvision cameras to firmware v5.5 or higher disables ONVIF protocol, causing cameras to go offline.

Hikvision's latest firmware - v5.5 - disables ONVIF capabilities in Hikvison cameras qulu uses ONVIF to discover, configure, and stream video from Hikvision cameras.
As a result users who upgrade their Hikvision devices to firmware v5.5 will see the following behavior:

• Registered cameras will go offline upon firmware upgrade
• New cameras will not be properly discovered

The Solution: 

Re-enable ONVIF, Add New User, Update Camera Settings

• Re-enable ONVIF Protocol
  • Navigate to your Hikvision camera web administration interface and look for the Integration Protocol section of the settings web-page
  • Click on the Enable ONVIF checkbox

https://ipvm.com/reports/hik-onvif-5-5

• Add a New ONVIF User
  • While still in the affected camera's web administration interface navigate to the camera's Configuration -> System -> Users menu and add a new ONVIF user (up to 32 per device)
  • Make sure the user has Administrator rights
• Update Camera Authentication
  • Right click on the affected camera(s) in qulu client and open the camera settings dialog
  • Update the Login and Password for each affected camera

Note: If you update all Hikvision cameras with the same user name and password you can update the Login and Password for all affected cameras at the same time by selecting all affected cameras and then opening the Camera Settings dialog.