Ok so I'm assuming this was originally filed concurrently with an I-485... when you have a pending Adjustment of Status application, you file under eligibility category (c)(9) and if filed on paper, it should have been sent to the Chicago Lockbox
USCIS Chicago Lockbox
U.S. Postal Service (USPS):
USCIS
Attn: AOS
P.O. Box 805887
Chicago, IL 60680
FedEx, UPS, and DHL deliveries:
USCIS
Attn: AOS (Box 805887)
131 S. Dearborn St., 3rd Floor
Chicago, IL 60603-5517
Once you have the tracking information you can request help by following these instructions:
If you or your representative believe a lockbox facility lost all or part of your package, please email Lockbox Support at lockboxsupport@uscis.dhs.gov and we will investigate.