Hi Eddy -
If Google does not have a targeted ad to show on your page, then by default a blank space will show in the spot where you wanted an ad to show. But you can change the default option if you want to.
In your AdSense account, go to Content > Ad Units > +New Ad Unit > Text & Display Ads. When choosing your ad type, you'll see this:
Choose what you'd like to happen to your ad unit if Google has no targeted ads available:
* Collapse the ad unit if possible, otherwise show blank space (default)
* Collapse the ad unit if possible, otherwise show a solid color
* Show another URL, such as an image.
I hope this helps!