That's a shame!
Well, we got it to the point where there are two possible interpretations left (aside from the odd possible case where *both* HDMI cables are defective... unlikely of course...):
1. you have a defective Digisender.
2. your Sky service is down.
I advise you to check on your Sky service, by calling Sky, asking them to verify that there is no service outage and your account is in good shape. (I hope I'm not talking nonsense here... I am presuming that Sky service is not just 'its taken out of the air its free' service. That it has a status and a company to call and so forth.)
If Sky service is not down, you probably have a defective Digisender I would guess. The light on the Sky box is green, after all. (Maybe that's just the box, not the Sky service itself, that is indicated as to its status.)
If the Sky service *is* down for now, you need to (have Sky) get it Up and Running again.
I see the case this way. If you have anything else to mention about the situation that might change my opnion, let me know, please.
(And, If this answers the whole problem/question of course, then please rate my answer if that's how it is.)