Many ads may not be available to you if you have not allowed personalized ads.

Please check your personalized ads settings.

Go to 

>Facebook settings

>Ads

>Ad Settings

>Then tap on 'Data about your activity from partners' option.

>Toggle the option on and see if you get your ads back.

If you have already turned on the option, that means there are no ads available at the moment.

The ads may not load when there is no ad available to watch.