Oculus sure knows how to keep people using their Rift headsets, with the VR giant recently announced through an update that Rift headsets going offline was caused by a lapsed security certificate.
Oculus Rift headsets worldwide went offline, stopping anyone from connecting to the Oculus runtime service, meaning that no one could use their VR headsets. There is a solution (something you shouldn't have to do, but you'll need to if you want to get your Rift back up and running) where you can set the date on your PC back, so that the security certificate thinks it is still active.
I wouldn't recommend doing this at all, and for now just raise your fist up at your Oculus Rift in anger. Sigh.