I am on both sides of google (pub & advertiser) hence I know roughly whats going on but here is the answer from the horse's mouth:
"We are implementing new brand safety controls that make it easier to manage where ads appear across all YouTube ad placements. We have temporarily paused auction Display Ads targeted to YouTube (GDN Overlays and Display Ads) to help us move faster on the infrastructure changes required to support brand safety changes for all formats.
We are actively working on the changes necessary to reactivate the format. We intend for this pause to be limited. We will confirm timelines as soon as possible."
This happened from the 21st March til now, and if you browse around youtube now, you would see almost zero display ads on all videos so its not only happening to you. There is no ETA on when they will put ads back on. This is related to the controversy that happened last week about the racist/extremist videos and brand safety.
Just hang in there & hope this helps!