Firstly I'd like to welcome you to the forums and may I say I'm sorry to hear that you're having some issues viewing the TiVo®, this could be down to a faulty HDMI port on the back of the TiVo®, in which case we'd need to wait for the technician to call out and take a little look at things. However on your other post, you did mention that you can see some of the menus etc, which is quite strange.
One thing I can think of is the video output of the TiVo® may need tweaking a little. In order to do this, can you follow these steps on your TiVo® remote:
HOME > HELP AND SETTINGS> SETTINGS> VIDEO OUTPUT> VIDEO OUTPUT FORMAT
From here if you go to Test formats, the TiVo® will give you a few prompts to follow, go through all of these and the TiVo® should highlight all the formats that your TV supports.
This may well help you out, let me know if it does.
The advice I gave will only affect the HDMI side of things, you would need a HDMI lead plugged in at the time, as when the TiVo® is testing the different resolutions, it asks you to press a button if you see the screen.
I'm not promising that this will fix the issue you are getting through HDMI, it's just a suggestion which could fix the issue, and in which case if it works, would mean you don't need to wait in for a technician.
Should you try this and it fixes the problem, please let me know so that I can cancel the appointment you have which will free up the technician for another customer.