https://www.polygon.com/2018/3/8/17096186/how-to-fix-oculus-rift-runtime-service-error-download

Yesterday owners of Oculus Rift headsets around the world woke up to find that their devices were no longer working. The reason? An expired security certificate. Today, developers at Oculus have issued a step-by-step guide to fixing the problem. Here’s what you need to do.

First, bookmark this forum thread on the Oculus website. That’s where the developers have been putting all of the information and updates related to this particular issue. If anything goes wrong, you’ll want to be able to find your way back here to troubleshoot it.