I had a document file (expn.doc) on "My Documents" folder in Drive C under Windows XP Pro SP2. Some days later when i opened that file, i found that the contents of the file were missing. Actually this is now a blank document file. I remember that one of my friend's used my computer for two hours for his emergency work. May be he opened the file and accidentally delete the contents of the file and save. There is another chance for him that he made a document with the same name and replace mine. After his completion of work he might delete data from that file and lastly saved.
However as you said the file have been overwirtten, then it is with no chance of recovery.
But there are also some options.
1, Your file was very large and it took many sectors to store and just partial sector has been replaced, then there is some chance that you can restore them, but only the not replaced partial file are recoverable. You may try some tool in advance, most of these tools are free to search files before the purchase like icare data recovery, getdata, stellar data recovery etc.
2, Ask some local data recovery service center to examine your disk and then there might be some chance of file recovery since the data recovery service center are more advanced than tools but of course more expensive.
Yes, your files maybe be overwritten. If so, recovery is such a difficult thing.
Do not do anything asap, and try a third-party tool with read-only nature and raw recovery feature.
You can find the top popular data recovery tool from some download sites, or find sth in this site.
I find a good choice here,www.tomshardware.com/search.php?s=raw+recovery, and the product page is easeus.com/datarecoverywizard
If your file is not too much, try its free edition because it helped me save my precious photos.