Watch out for new updates to Shopify themes. It is possible that your classic theme has become a 2.0 theme. So follow the 2.0 theme steps if necessary.
1- Go to your Shopify Themes menu. (online store -> Themes)
2- Click on the "Actions" menu, then select "Edit Code".
3- Open the section you use for product pages (usually this is called product-template.liquid or product.liquid, but some themes work differently).
4- Copy the line of code below:
5- Paste it where you want the stars to appear on the product page (we recommend pasting the code under the product title), then save your file.
<a href="#trustreviewsFrame"><div class="trustreviews-rating" data-id="{{ product.id }}" data-rating="{{ product.metafields.trustreviews.avg_rating }}" data-raters="{{ product.metafields.trustreviews.num_reviews }}"></div></a>
Here is an example on the Debut theme: