Sometimes, I noticed that my Rift isn't working. It's constantly on and the PC sees it as another monitor, and not as a gadget. I searched the web, it seems to be a quite common problem, with no concrete fix. I recently got it to work again and this is how I did it. Note that I have an Intel processor with an AMD graphics card and I use the CV1 Rift.
I noticed this is always happening after Windows had a mandatory update. I guess something internal switches, and the graphics card views the Rift as another Monitor, and not a USB/HDMI Peripheral. Anyways, I tried these things and they did not work, but might work for someone else:
-Restarting the PC
-unplugging the Rift on both sides
-Use the "Repair Oculus" option
-Undo the recent windows update
-wait until it magically works by itself again
-make sure that your PC isn't running on a power save option, which could turn off USB slots from time to time
-People said that some PCs have an option in the BIOS that is dedicated to USB slots, I don't know enough about it, but you could check it out to see if there is something off
-Reinstalling Oculus drivers
-Deactivating the Rift in the Device Manager and activating it again.
All of the above didn't work out. They might have helped, but the problem didn't disappear. What really helped, was installing the newest drivers for my graphics card, even though I was already having the latest ones. I could suggest trying that. Just reinstall your graphics card drivers and restart your PC.
If you also had a "No HDMI Connection" problem and were able to fix it, please share your tips, it could help people out, since Oculus themselves don't seem to be really helpful in such a situation and the problem could have many causes.