Being out of the country doesn't necessarily forfeit your permanent resident status, but leaving the country to establish residence elsewhere for more than a few years or so could well do.

The Consulate is at least a place to start. Unless you've got a spare $300 - 400 to engage an immigration attorney for what could turn out to be a wild goose chase. A visa doesn't guarantee you'll be allowed to enter the US, either, but it's step one in going over there.
Cheers,
Bev