Is this a pre-built under warranty? If so, doing this yourself may void any warranty left, if it would even cover this.
Open up the case. Check to make sure the cooler is still on the CPU, the RAM and graphics card(s) are fully seated in their sockets all plugs and wires are fully connected and not partially. Also check for physical damage to the motherboard, if it's damaged, the damage should be seen under bright light. Look carefully around the CPU cooler on the motherboard. Also check the expansion slots, in particular the one(s) that contain(s a) graphics card(s) too. You'll be looking for anything from cracks to obviously broken off pieces. There's always a chance your HDD might have been damaged (internally) in the fall too. However, if it was you'd probably hear it on startup.