-Do you often travel outside US or stay outside US for a longer time? If so, it may be the reason.
I suggest you to go to the local CBP Deferred Inspection office to see why your green card was taken away at the port of entry. Then, you can decide whether you can hire a lawyer or not. Normally, it is difficult for them to cancel your permanent residency because you still can appeal in the immigration court. So, not need to be panic at this point.
See this website to find the local CBP Deferred Inspection site in your area:
http://www.cbp.gov/xp/cgov/toolbox/contacts/deferred_inspection/