Well, this is a two sided issue.
One side is dependent upon any recent apps you might have installed. Sometimes an app you install may suddenly incur pop up ads in your notification bar. If that is the case, find out which one and uninstall it. I have had to go down that road several times.
The other side is that if you root your device and find an ad blocker app (all of them require root access and most [if not all] are not found in the Play Store), so you are left to your own Google search or other search; then you can block the ads that way.
good luck