The source – ads.google.com
The ad system's troubleshooting page has posted news about a glitch in Google Ads. It affected a "significant number" of users and affected the display of ads in the desktop version of Gmail. Now ads may not be displayed for some time.
To name the exact terms of elimination specialists can not yet, but promise to eliminate the problem in the near future.
You can track the status of the incident on the original source page.