Dropped while running. It's possible that the hard drive got knocked loose. However, it's more likely that the HDD has failed. It's the only thing that had moving parts when it fell. If your still under warranty, contact Gateway if your still within the year, or Costco if they offer some extended warranty. Don't tell them all the details, just tell them it's not working, and it will be covered.
If it's not under warranty you'll be looking, at minimum, a new hard drive and reloading the Operating System.