SO, I did the same exact thing. I believe the reason is that you have update to 4.2.6 from 4.2.5 that the phone shipped with. I haven't tried it yet but I'm going to right now.
I had the exact same problem. So, i waited forever for 4.2.6 to install, and then tried greenpois0n again. when it turned on, the loader opened and i got cydia to install just fine