Most likely, the campaign is not getting traffic for one of the following reasons:
Your ad creative was rejected. Check it out in the campaign settings.
You ran out of funds. Campaigns stop receiving traffic when the account balance has been depleted.
Your bid is too low. Try to raise your bid. Dynamic CPM will help you to bid at the best possible price without exceeding the bid you set.
There is no available traffic for the selected targeting.
The settings of your ad campaign are controversial (e.g. you target a mobile device but have chosen a desktop operation system or desktop version of the browser)
You’ve set up a schedule or an end date for your campaign, please check the scheduling settings and the time zone
Compliance flags limit traffic specifically from the chosen ad spots. You need to check the creatives section and see the list of the flags
If none of this helps, then contact your account manager.