In order to sign into my blog, I go to my homepage and log in from there. Today for the first time I saw a banner appear over the top of my site, hiding the name of the site with an advertisement for a girl earning 450 US Dollars without a job. Kindly pay attention to it asap?
WordPress.com does display ads on free sites. They help to keep free sites free.
Banner ads at the top of sites can be dismissed by clicking a button. If you think any ad is objectionable there should be a small “report this ad” button you can use.
Upgrading to a paid plan is the only way to ensure your site remains ad free. https://en.support.wordpress.com/no-ads/