DO NOT keep trying to fix it. It's a known issue related to the Slingbox itself, a defective power supply, or a recent firmware update, depending on which thread you read.
The same thing happened to me this evening, I unplugged/replugged a few times and all of a sudden, SMOKE started pouring out of the Pro-HD. So I've currently been on HOLD FOR 56 MINUTES with Sling support trying to get an answer.
Here's the thread that clued me in that it's not just me: https://community.sling.com/message/66363
Hopefully sometime soon tonight I will get a human being on the phone who can help me.....I have another Slingbox Pro-HD in a remote location that I can't connect to also. I hope that one hasn't set anything on FIRE.