It is extremely difficult to predict what data can be recovered and what cannot, but users who lose data due to a mechanical hard drive failure can often recover most or all of it. Deleted files are often recoverable as well.
Hard drives have a number of moving parts, and some of these parts can break without damaging the data on the hard drive. For other problems, data recovery works by searching for files on the hard drive. File systems store a list of locations where files begin, and deleting files often means simply erasing these pointers and marking the region as empty space. If another file hasn't written to this portion of the hard drive, recovery software can often restore the file.