If you know what theme you are using, open up the archive of the original, find sidebar.php and open this in an editor, notepad will do.
If you have not made any other modifications to your sidebar file that you are worried about losign then just copy all the contents of sidebar.php from your original install archive.
Browse to Appearance > editor in your WP admin. A list of theme files should display down left, click sidebar.php and paste in the code from your original.
If you have made modifications you wish to keep you will need to identify the code that needs to be replaced and only paste that in.
Hope that helps.