I'd call the main Customer Service USPS line or search online with your city to locate the closest office (the local office may direct you to the main office that actually oversees mail delivery for your area*).
We stopped receiving mail at our weekend residence one day. There was also a rubberband placed over the door of our mailbox. It was strange. After three weeks of non-delivery (we're talking junk mail too, so it was more weird as that was all we'd get), I called the office that delivers to our area and spoke to someone. She insisted we were still listed as active for mail delivery and had no idea why there would be a rubberband over our box. The next week we still didn't receive anything and the rubberband we had removed was back on the mailbox. This time I went into that office and spoke with I believe the same woman I talked to on the phone. She went in the back, and when she came out again she discovered that the letter carrier assumed our house was vacant so he stopped all deliveries. I had to complete another card to initiate service at our address, and we've had no problems since then. Of course now we make sure not to let three weeks go by without going to our house (we had been on a long vacation before so wasn't at our cottage for a while) in case the mailman decides we don't live there anymore.
* Our house is in the Village of XXX but the neighboring city of YYY delivers our mail even though we have our own post office.
Oops, as I was typing, someone else answered more succinctly.