You mentioned that you changed themes. Were you using a WordPress ads plugin like Advanced Ads or Ad Inserter? If so, you might want to go back through the ads and settings in the plugin to be sure nothing is wrong there.
If you did not use an ads plugin to place your placeholders and they are missing entirely from your posts, you might consider using the Chrome Ezoic extension to double-check the placeholders have indeed disappeared.
You could also go to the Dashboard > Monetization tab and double-check your placeholders there. If they are missing there, you might consider adding them back using the Chrome extension. If they are not missing, and are showing up in the Monetization tab, go through those that you set for posts and be sure no settings have changed for each placeholder. For example, be sure nothing weird has happened such as the placeholder not being turned on for different devices, etc..
Finally, see if the ads are missing on mobile and desktop. If they are showing on one and not the other, it could be an indication that something is set wrong in the placeholders as I mentioned in the previous paragraph.
A support person could probably shed more light on the subject, but this is where I would start if I were in your shoes.