installed plainvanilla theme and keep getting this error

Did you upload the header file?

Looks and reads like you put the theme index file in the root directory instead of in its own theme directory and overwrote the index file in WP’s root. That index file should not be overwritten as it calls the blog-header.

just upload the files that are missing from the wp root. Remember to put the correct wp-config.php with your database info

your blog is always safe if you aren’t tampering with the database