srikakulams

Hi everyone
I have been given immigrant visa in July 2007. I entered the US the same month and got permanent resident status. But i never the received the so called greencard....when i contacted i ve been told that the card was posted to my old address...and so that got returned and reposted to new address...but yet i didnt receive it...now i am asked to apply for i-90

Meanwhile i had to come to India as i am still studying here. My immigrant visa expired in december 2007.
Now can i enter the USA without the greencard in my hand? How many months can I stay outside USA without another entry????

Please help me with your suggestions...Thank you.