Please note that the USPS is sometimes marking packages as delivered a day or two before they are actually delivered.  If you wait a day or two it is quite possible your package will turn up.
If you requested a signature on delivery there should be proof of delivery in your USPS tracking information and a copy of the signature obtained to assist you in tracking down your package.
If the package was sent with no signature required locating your package may require a bit of detective work. (Customer assumes responsibility for packages sent with no signature required once they leave our warehouse.)
First, make sure a notification has not been left by the carrier. If not, then please check with any household members, neighbors, or office staff who may have taken the package in for you as well as any “hidden” spots where a package may have been left for you.
If the package is still not found please contact the local office of the carrier, (ie, your local post office, local Fed Ex hub, etc). They would have been the last people to have possession of your package and may have further information for you. Once a package is picked up by the carrier it is out of our hands and we have no further information than that provided in tracking information.