Britsimon is right, though this may of course also apply to green cards received through the DV...
No expert but from what I understand the presumption is that you have abandoned your GC (abandoned residency) if gone for longer than a year. Hopefully you have enough evidence to convince an immigration judge that you haven't, I understand this includes things like having maintained a home in the US, having filed tax returns with the IRS even while you were gone, and so on. But if you are going to court presumably you have a lawyer who will be advising you on this.