If you have lost some of your important data, you can recover the data is a two step process - recovering the data from the unallocated segment of the media with unrm and then reconstructing it with lazarus. You may try to read the main pages for unrm & lazarus, or the file "lazarus.README", which will go on and on about how lazarus and unrm work. Failing that, this is a perhaps kindler and gentler approach to the problem.
Determine the file system the lost file was on. For those not familiar with this concept, try typing (the "%" is the prompt - "#" is used when it must be done by root):
% df /directory/where/file/was