As a green card holder you need to reside in the US for 183 days each year to maintain your status. If you will be absent for two years you should get a re-entry permit. In your case, your green card is probably revoked. You should fill a form DS117 in order to apply for a SB1 visa the nearest US embassy. Document the length of your stay, US tax returns you filed, and the reason why you could not return in time. If your visa is approved and you are allowed to come back in, then your green card is reinstated and you should renew it immediately. Consult an immigration attorney.