If the HDD is OK, it will show "House Keeping" for about 5 minutes and then reset it's self. It will be on stand by, RED light on the front. You press the Sky button and it should come on like it's just been reset (i.e. looking for channel etc.)
If when you did the full reset it started with the "house keeping" message and then went blank after about 10 seconds. I'm afraid it looks like you have the same problem I did. A faulty HDD. You can do a power reset and get it to come back on and use it as a normal sky box. At least yours in the warranty period, mine was 2 years old and I had the pay for the new HDD myself.
I am sorry that this has happened. Don't be too dishearted. Sky+ is a great toy, I couldn't be without it.
Too Tall.