A malfunction in a hard disk drive. There are two types of hard drive crashes: the first is physical, where the mechanism of the drive fails. In this case, as long as the platters are not damaged, there is a good chance that data on the drive can be recovered. If the platters are physically damaged, the data may be irretrievable. Physical crashes are usually signaled by odd noises coming from the drive, or a failure to spin up when the unit is powered up.
The second type is a “logical” failure, which may be caused by a virus, software bug, operating system/driver incompatibility, or other problems. Depending on what caused the crash, the data stored on the drive may or may not be recoverable. Typically, with a logical failure, the drive will spin up, but the computer or other device attempting to access the drive will be unable to mount it.