Exactly what happens depends on the nature of the crash. The heads dropping and hitting the platters of the drive is never a good thing. That generally is a problem that cannot be easily repaired, if at all. The tolerances are very close. Best thing there is to stop using it to minimize any damage, and if you are inclined to have someone try and recover the data, give them the drive to have a go at. But that type of recovery can be very expensive and there are no guarantees.
There are probably as many ways for the drive to fail as there are parts in the drive. "Crash" is just a generic term that is applied to the problem. You may be able to have one repaired, again depending on what really happened to it, but drives are getting so cheap nowadays. Best Buy has a 1TB (terabyte) drive advertised this week for $129.
I have an EHD that recently just quit working. I think my problem is something with the interface of the enclosure and not the drive itself as my being able to "see" the drive would come and go. It was preventing me from booting my PC, and if I booted with it unplugged, my PC would start up and then I could plug it in. At that point, sometimes I could see it and sometimes I couldn't. I am planning on moving it to another enclosure to see if that really is the problem or not. I replaced it in the mean time with a 1TB WD EHD that I found for $129 before the holidays.