Probably
If it's just the TV and Sky box...
Your TV is the likeliest culprit. Impossible to advise specifically, but you'll need to go through its set-up menus. The remote should have a menu. On mine (Pioneer) there is a choice labelled 'option' and HDMI is set in there. (Might be worth cehcking if Sky box has SCART control set to 'off').
I would expect yours to be similar.
If you have an AV system with an AV amp controlling the digital signal switch, that might add another level of complication, but assume the TV first, then look for something similar on the AV amp set-up menu.
If all else fails, try web site of TV manufacturer and see if they have a FAQ section or help facility.

))
Have fun!