Earlier this week, Oculus Rift VR headsets around the globe unexpectedly stopped functioning, returning error messages and refusing to boot up. The issue affected most headsets around the globe over the space of a day, and seemed to catch Oculus rather unprepared.
Users soon worked out that the issue was a software certificate expiring without Oculus noticing, which meant the headset was unable to be updated the normal way - which is why issuing a fix was more complex than usual.
Today a patch has been made available for Rift users, which will allow them to return functionality to the headset. You can find the patch, and instructions for installing the fix, here. If you've been unable to use your headset as a result, hopefully this fix will get you back up and running.