I ran DBAN (Darik's Boot And Nuke) on my notebook. When it finished, it showed a message saying "press any key to continue". When I pressed a key, I got an advertisement page, and then it stopped.
Has my hard disk been wiped completely?
Yes, it's wiped completely. I don't remember that it had an advertisement. Since the last time I used it, maybe they changed something because it's free.
The accepted answer is correct.
However, to throw some more light on the problem, I remark that DBAN was purchased by Blancco in 2010, which is when the ad was added.
An examination of the SourceForge DBAN page shows that the ad was added in version 2.2.6 and also that some useful development was done at that time of beginning 2012.
As no development was done since beginning 2012, it might be worthwhile to look for an alternative product.
DBAN itself is criticized in the article Securely Erase A Hard Drive – DBAN May Not Be Sufficient as not actually wiping the entire disk.
I remark that the dwipe program that DBAN uses has been forked and is available as a standalone program called Nwipe, included with partedmagic (if you want a bootable CD version).