Oh dear!
This is a known issue after the SkyHD EPG update shenanigans, as it appears at first glance to change the default transponder settings.
If you don't have a really good signal on this particular transponder, you will experience the symptoms that you are describing.
Try the following exercise, it appears to work for some but not all . . . .
1. If at switch on it says no signal and a number in the video window box then do the following.
2. Press the Guide button.
3. Go to the top (****tions) bar.
4. Scroll through to the Settings menu.
5. Scroll down to Picture.
6. then press 0 then 1 then Select after each other to get to installer menu.
7. Scroll across to TX`PONDER.
8. If it says 11.856 V change to 10.714 H.
9. Exit, then reboot.
Depending on your signal levels, it may just do the trick.
Good luck!