[AZ-AlphaXi]

If you were a member of an NPC sorority then you may be out of luck. Some NPC groups allow for re-instatement after resignation after graduation, some allow for re-instatement while still in school, some do not allow for re-instatement at all.
If you were an initiated member of an NPC, you cannot pledge another NPC at any time.

My advice to you is to contact your (Inter)National headquarters and ask what your options are. If you are allowed to re-instate, there will probably be a re-instatement fee.