It sounds like your TiVo, which has an inbuilt modem, has lost it's internet connection.
Using your remote go to Home > Help & Settings > Settings > Network connection. When was the last successful connection?
Click on "connect to Virgin Media service" and re check in around 30 minutes. What does it say?
Hopefully it will be connected and your EPG will be populated, along with the missing channels.
Also, using your remote control you can force update the box to search for missing channels by going to
Home > Help & Settings > Help > Virgin TV > go up to the top of the screen then TV Care and press OK > Missing Channels.? > Refresh my channels list
Dave
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali