If immigration finds you've been out of the country for more than 2 years straight, you lose your residency and they will confiscate your green card. If you were gone for more than one uninterrupted year but less than 2, you were supposed to apply for a re-entry permit- don't know what they'll do if you didn't have one, but it's probably not good
If you've been making visits to the US so that you haven't been out of the country more than a year on any one trip, they'll take it on a case by case basis...if you've spent most of the year out of the country, it'll look suspicious. If they believe you've abandoned your residency (ie you don't have a permanent US address or you've failed to file US taxes while you've been away), they will take away your green card. Don't know about crossing the land border, though...never done it myself, so can't help you there.
I'm not a lawyer or immigration advisor, just a green card holder myself who's read and researched extensively on these kinds of things
hope this helps!
