Hey everyone, we had the "HDMI not found" issue and found the solution...
so - the computer is a Predator, pre made purchased from a major retailing store in Australia, the specs are good for a gaming PC - Radeon rx400, Intel i7, 16gb ram SSD HD etc. We were using the rift to play around and went on holidays last year around August 2019, came back, turned on the PC - it did a few windows updates and then the dreaded "No HDMI found" issue. We went through the process of the support through Oculus and they wanted the headset sent back - we did what they asked, and a new Rift turned up a week later. Plugged it in, followed the setup process - and no HDMI. Sent back the Rift and received another, and again "No HDMI"
We decided to trouble shoot the problem and we decided it was a problem with Radeon drivers, so downloaded all of the drivers and the driver removal tool and went through the last 5 drivers, still no HDMI. So, not a driver issue, the Rift is brand new, must be the graphics card......... Purchased a RX5700xt and put it in, installed the latest driver/software package 20.4.2 - tadaaaa, it works no issues at all so removed the RX5700xt put in the RX400 and no HDMI issue apeared again ......... looks like the MSI Radeon RX400 is not supported from September 2019 - can only assume that the minimum requirements have been updated and not advertised
thinking this might be an issue with NVidia cards too
hope this helps someone