Re: paused display campaign when I activate does't show any ads
Could you check the drop down under the Ads tab (screen grab below), if you've selected the right option. May be you have paused or deleted ads in the campaign and you're looking for them under 'all enabled' ads. In that case, you can select 'All' to see all ads including paused and deleted ones. Same goes with placements as well.
Let me know if that doesn't work.