Both FedEx and Ups have a security department that specializes in the investigation of lost and stolen packages.
If you know your package was stolen you need to both contact UPS or FedEx and the police department to make a police report about the stolen package.
If the package was stolen the UPS or FedEx security department will work with your local police department on finding who stole your package and hopefully recovering the package.
There's no guarantee that the police can recover your package but there's a chance.
If the package is lost in their system they'll do all they can to find the package in their system and if found they'll report back to you and send the package to your address or allow you to come pick up the package.
If the package cannot be found then usually the person or company shipping the package will be paid back through insurance coverage on the package.