Which browser are you using? It makes a great deal of difference because different browsers store bookmarks in different ways.
In Internet Explorer, your bookmarks are stored in this folder:
C:\\Windows\Favorites.
Each bookmarked site is stored as a separate shortcut, with no back-up copies. If you're using Internet Explorer, take a look in that folder and check the contents. If your bookmarks aren't there it's possible that they might have been lost forever. However, try using System Restore, to see if they reappear.
In Firefox, bookmarks are stored as part of a single HTML page. One of the (many) reasons that Firefox is far better than Internet Explorer is that it backs up that HTML page. If you're using Firefox, run a search for 'bookmarks'. You should find two files called bookmarks.html. One should only be 7KB. (That's the one which Firefox is shipped with). The other one should be your current Bookmarks page. There should also be some files with recent dates on the end of the file names. (They're recent back-ups). If one of them is bigger than your current Bookmarks file, that's the one you want to use. Here's how:
Right-click on the dated file which you want to use. Select 'copy'
Right-click on the current Bookmarks file and select 'Open containing folder'
Within that folder, right-click and select 'Paste.
Right-click on the current (undated) file and select 'Delete'.
Right-click on the other (dated) file and select 'Rename'.
Rename the file to 'bookmarks.html'
Start Firefox and your old bookmarks should have reappeared.
Chris