If it's not at your door, not at the apartment office, and UPS is adamant that it was delivered, then the package was likely stolen. It's unfortunate and one of the problems with living in an apartment complex.
This happened to my wife last year. She ordered some clothes, UPS left the package at the door (they are supposed to leave everything at the office if no one's home), and someone stole it.
We contacted the company, filed a claim, and after a a week or so, they shipped out a replacement package.
Are you able to have packages shipped to your workplace? This is what I do for a lot of stuff. The bigger items I will have shipped to my parents' house or have my parents check the office for me (they live right around the corner) since I take the train to work.
I contacted the UPS store directly, they say they wouldn't leave it on the front door and would leave it at the front office. They say to contact CS, and now CS is saying contact them directly. Fuck this, that's my $356 down the drain.
Nah, you'll be fine. Contact the store you ordered it from if UPS isn't helping.