Sounds like a stuck spindle. Data recovery services are not cheap, but they can get your data back if the platters are intact. If your data is really important to you, don't do anything else with the drive yourself unless you're unable / unwilling to send it in for professional recovery.
The heads might have crashed, hopefully not while over anything irreplaceable. Laptop drives have sensors that will park the heads if they detect a drop, but I don't know that desktop drives or drives used in external enclosures for desktops have that capability.
You might be able to get the spindle unstuck yourself, but you'll be risking further damage to the drive and your data.
The "bag it and put it in the freezer" trick seems to always be suggested in these cases, and indeed, it has here, in comments and other answers. If you're not going to send your drive in to a recovery service for whatever reason, I suppose that's an option. However, I don't suggest that you randomly try things without a bit of research and planning first. Ask yourself what the theory is for the solution you're about to try, how it will get your platters rotating without further damaging your data. If the answer isn't convincing, I wouldn't try it, unless you're out of other options and are ready to dispose of the drive and your data.
If you're prepared to void the warranty on the unit (if dropping it while operating didn't do that, even if you did catch it before it hit the floor), you can pull the drive from the enclosure and see what make / model it is. That might suggest some drive specific remedies. Also, as others are suggesting, it will rule out damage to the enclosure cabling.
There seems to be a cottage industry of more economical data recovery firms springing up, with tools even available. Youtube is also awash with home HDD surgery videos. Again, I wouldn't try any of this yourself unless you're otherwise prepared to give up on your data and trash/recycle the drive. Your drive is almost certainly multi-platter due to the capacity, which makes something like a platter swap very difficult, since the platter alignment must be maintained. I've seen reports that some drives (Hitachi) have calibration data in NVRAM on the the logic board, which makes mechanical transplants useless unless you move the NVRAM data, too. I don't have a link handy at the moment, though. Something else to keep in mind, assuming non-clean room repairs / swaps are possible, what are the odds you'll be able to pull it off the first time without making a mistake? How much practice would you need?
