If your website's ranking has dropped and the on-page title is different from the title shown in search results (SERP), there are a few steps you can take to fix the issue. First, make sure that the on-page title in the HTML matches the title tag and is relevant to your content. Next, check for any recent changes on your website that might have affected SEO, like updates to content or design. Finally, consider improving your website's overall SEO by optimizing keywords, improving page speed, and ensuring mobile-friendliness. After making these changes, monitor your rankings to see if they improve.