Since Adsense ads in Google Sites were sun-setted, under certain circumstances if you make changes to your Site's theme then your ads may no longer display.
For a site that has Adsense ads, the ads are defined by a set of parameters like size and colours. The colours actually derive from the Sites colours (that are largely based on the theme) and the option to have the ad blend in or stand out. All existing ad configurations (per site) have been keyed by these parameters and saved in the Site configuration. If you change something in the site that would require a different kind of ad (e.g. with different colours), you will not be able to generate an ad.
If you revert to the theme/settings that were previously current, the ads should start being displayed again.