1.) The most straightforward approach is to restore a backup.
2.) Try data recovery software. There are several applications out there which can do the job for you almost automatically. Please be aware that these apps might be expensive, but worth the money if you've no ther options!
3.)If the data is critical and you cannot live without it, you can send your hard disk to a data recovery company. They will do an in-depth analysis and try to restore your data -- even if it is already overwritten. This is the most expensive option, so consider it carefully!
This was first published in September 2002