My sister owns a HP laptop. Recently, while powered up, she knocked it off an ottoman. This caused a head crash and intermittent clicking noises indicating a broken hard disk. When powered, the drive responded, but would no longer boot into Windows. I put the drive in an external enclosure and was able to quickly backup all of her data. The majority of the data was luckily intact and accessible.
The most important thing to remember about hard disks is that they will fail. Often, they will do it without warning. So you must make back ups and keep them current.
Recently, I had a drive fail with excessive bad sectors. It happened after I had moved a new set of photos off my memory card, but before I had copied the files to my backup drive. The bad sectors corrupted the directory structure on the drive, but I doubted the actual file data was effected. It was time for a PhotoRec recovery.