Frozen Fork on 2011 Diamondback MTB
According to Performance, your fork is a  "SR Suntour XCT with 100mm travel, 28.6mm stanchions". 

These aren't really forks designed to be serviced -- they're basically the bare minimum of forks which are not completely for fashion statements. You can look on Youtube for videos of people trying to service them and what not -- you're basically going to strip it down as much as possible (which may require some penetrating oil), grease it up, and put it back together (you might get a head start on this by banging it a bit to see if it unsticks). Note that this is  maybe a 50 dollar fork, and depending on how much you value your time, you might want to take it into a shop to see if they can unstick it or just replace it, possibly with a rigid fork if you're trying to save money.