My green card wasn't taken off me when I left New York last December. I didn't realise to start with but when I did I contacted US Immigration (via the US Embassy). I then had to send my green card along with a letter from my employer confirming I was back in this country along with my boarding pass and a copy of my passport to a US Immigration address in Kentucky. I have to keep a copy of all the documentation I sent to them in case I have a problem next time I try to enter the US. They don't apparently acknowledge correspondence nor write to you to confirm that everything is in order hence having to keep copies to show immigration next time in case it hasn't been smoothed over. So yes, your first step is contacting the US Embassy who give you a phone number for US Immigration who will give you the address to send all your stuff to.