Displaying sidebar on Nature template

I’m not familiar with that theme, but suggest you download the theme again (to your computer, not inside wordpress) so you have a fresh copy of the files to compare ith yours. Do you remember what file you were editing before the sidebar disappeared? If it was index.php check near the bottom of your themes index.php file with the one in the zip and look for something like this:
<?php get_sidebar(); ?>
If it is missing from your themes index.php, then add it back to the correct spot and you should be good to go.