Get another monitor and check the output of the TiVo. If it, too, is blank, then it is probably an issue with the Tivo. If not, then the problem probably rests with the TV.
If both are blank, record one of the blank videos and see how large it is. You can log in to the TiVo across your LAN wiht any web browser. Just enter
https://<ip of your TiVo>
When it asks, enter a user name of tivo and enter your MAK as the password. You should be able to see how large the files are, and (if they are not copy protected) even trransfer them to your PC.