You were using your drive (PC running Windows Vista) and while it was running you banged it (was on the edge and then it fell onto the flat side). At that point you unplugged the USB cable and then reconnected (failing to dismount the drive). Now you can't access the drive, but the device driver and the USB interface internally of the drive is responsive (drives power light is lighting). So the drive was spinning when you knocked it. And, you failed to dismount the drive correctly.
We don't know if the system was still in the process of writing to the drive (where you in the process of reading or writing a file off of the external drive?) If not then the cache write back is only effected which is a minor issue. Otherwise the file you were accessing is likely damaged.
The big issue here is what was the drive doing, was the disk head arm parked or engaged? From the sounds of it it appears the arm was engaged so it was somewhere on the plater/s. So the heads likely dug into the platter ripping one or more off and damaging a part of the magnetic surface of the plater (heads crashed). It doesn't take much for this to happen.
OK now what - If the data is that important you'll need to send your drive in to a data recovery service so they can disassemble the drive in a clean room to try repair the drive long enough to pull your data off (replacing the damaged heads).
If you have also encrypted your data you will need to give them the application and the encryption keys so the they can decrypt the data. It also turns out the HD it's self is hardware encrypted so it will only work with the logic board it is mounted to, so you can't send them just the hard drive internally alone either (need to ship the complete drive with the power adapter).
The cost will be expensive and there is still a chance they can't recover the data. Attempting to fix this via software won't work and if you are able to get it to respond the process could make things worse. Here's a [http://www.pcmag.com/article2/0,2817,2361118,00.asp|PC Mag article on what happens in a clean room] Don't forget to try out their drive simulator to see what happened to your drive.
So This is what we know:
You were using your drive (PC running Windows Vista) and while it was running you banged it (was on the edge and then it fell onto the flat side). At that point you unplugged the USB cable and then reconnected (failing to dismount the drive). Now you can't access the drive, but the device driver and the USB interface internally of the drive is responsive (drives power light is lighting). So the drive was spinning when you knocked it. And, you failed to dismount the drive correctly.
We don't know if the system was still in the process of writing to the drive (where you in the process of reading or writing a file off of the external drive?) If not then the cache write back is only effected which is a minor issue. Otherwise the file you were accessing is likely damaged.
The big issue here is what was the drive doing, was the disk head arm parked or engaged? From the sounds of it it appears the arm was engaged so it was somewhere on the plater/s. So the heads likely dug into the platter ripping one or more off and damaging a part of the magnetic surface of the plater (heads crashed). It doesn't take much for this to happen.
OK now what - If the data is that important you'll need to send your drive in to a data recovery service so they can disassemble the drive in a clean room to try repair the drive long enough to pull your data off (replacing the damaged heads).
If you have also encrypted your data you will need to give them the application and the encryption keys so the they can decrypt the data. It also turns out the HD it's self is hardware encrypted so it will only work with the logic board it is mounted to, so you can't send them just the hard drive internally alone either (need to ship the complete drive with the power adapter).
The cost will be expensive and there is still a chance they can't recover the data. Attempting to fix this via software won't work and if you are able to get it to respond the process could make things worse. Here's a [http://www.pcmag.com/article2/0,2817,2361118,00.asp|PC Mag article on what happens in a clean room] Don't forget to try out their drive simulator to see what happened to your drive.
Sorry for the bad news [-{