Data or File Corruption
Data corruption is caused due to programme or software crash of failure. A simple power failure can sometimes cause Data corruption. Other factors that contribute to Data Corruption are software built-in bugs and hacking. In these cases, a good hard drive recovery may not completely serve the purpose. Data recovery specialists can do this job better since in these cases it is always good to identify the root cause of Data Corruption. Standard IDE/SATA are more prone to disk failure and corruption due to the mechanical nature of the environment. Solid state drives are much more reliable in this regard but they are too expensive.
File corruption can also be caused by OS crash or partition table crash. In most cases of OS crash, the user files will remain untouched. Hence booting the system with external USB or Optical drive with another OS can help easily recover these files. Partition table can be recovered using a good partition table editor.
Accidental File Deletion
Not sure if you knew this already but any file that you even permanently delete [Shift+Delete] is not being actually deleted. It simply stays on your drive until it gets overwritten by some other file. Of course, the permanently deleted file will lose the address from the Storage Index but it still remains in the drive. Windows NTFS systems make it easier for file recovery as it stores file description information even after the file is deleted. Other file systems like FAT, UTF will delete all metadata related to files like the file description and path information.
If you take a quick action to recover the file you stand a higher chance of getting the file back intact. File recovery software use complex algorithms to find the deleted bits and pieces and gather them up to a single readable file. The only concern is doing any activities on the computer can result in removal of these stored bits and pieces. Even browsing the internet can result in overwriting of these deleted bits with the new cookies and cache which your browsing activity is generating. If you have ever overwritten the file with new data, no file recovery software can help. So if you badly need to recover the files, stop using the hard disk by all means.
Physical Drive Damage
In cases of Data Corruption and File Deletion, a good file recovery software can serve your purpose but a damaged drive is best dealt by professionals. It also requires the drive to be taken out of the computer and this needs to be done in a clean place free from environmental pollutants. A physical damage to a disk can be caused by several factors including the crashed head or broken controller board. Data recovery, in this case, might be a little more expensive as it might include rebuilding some important parts of the hard drive, developing a clean disk backup images and recovering corrupted partitions in whole using sophisticated tools.