overstaying with student visa
 

I' d be very gratefull if someone can help me on this:
On March, 1996 I went to US with F1 Visa. My VISA was for 1 year and two months. I was in a University for 1 semester. Then I stayed without trying to extend my I-94(even did not know what period of staying was given). On September, 1997 at the airport I didn't turn in my I-94(because I fonud out that I have lost it-). I show the person at the departure-my I-20 form and a student card( that I attend the university) Then the person told me I can't go back to U.S.
Now I'm planning to go to U.S for my Ph.d
My question is:
*How can I find out if I'm baned for going back to U.S
*For how long could the ban be

Thanks a lot