Sounds similar to what my Premiere did when the hdmi port went out on it, to test you need the AV breakout cable, you might be able to find one locally, Bestbuy would be a place to start or if you aren't in a hurry you order off the Internet.
I wouldn't think you would lose anything if you swapped your original drive back in which you would need to do if you make a warranty claim. Problem would be if Tivo replaces the Roamio, it then that might take be questionable if the 3tb drive would have the recordings on it, if you can back it up by your PC then that might be the only way to keep your recordings.
Do you have kmttg or Tivo desktop installed to see if you can access the box on the network after waiting awhile to be sure it's up? If so then that would suggest a video problem.
Might also try radio Shack too for the cable