I was tweaking one of my campaigns recently and it feels like the reason it has now stopped running could be from the vast list of negative keywords I recently added. The campaign was running on the Content Network, and was a typical Banner Ad for PricelineEurope.com, and was receiving several 1,000 impressions a day for about 65 cents a click. So can adding too many negative keywords stop this campaign from running? Should I use one-word keywords or two-word keywords ideally? How many negative-keywords should I use for this type of campaign? What would be good choice of negative-keywords for this type campaign? Does adding a keyword such as 'Work' eliminate my ad from appearing for the synonyms like 'business' as in 'business-class', therefore a large keyword list eliminates an ad from ever showing (because all synonyms would be a synonym)? How do negative keywords effect the Content Network, Search or Placement Networks differently?
I can see how having too many negatives can stop an ad from running in the search network. However, I wasn't sure of the impact of negatives until your question. I thought they had no effect. But the following link says they do: adwords.google.com/support/bin/answer.py?hl=en&answer=98426 So, yes, having too many negatives in your content campaign could stop ads from running. As for one-word, two-word keywords, I never, ever use one-word keywords. They just don't describe your offer at all. Two-word keywords are better but often the same keyword can have many different meanings and they still are iffy at describing your offer. So go with three-word keywords or longer, which by the way, will reduce the number of negatives needed. I think your problem is using single-word keywords. Personally, I'd use the placement network and target specific sites for the kind of offer you have.
I agree with what Lucid had to say and like to add few more comments: 1) Double-check your adgroups are themed correctly. Create multiple adgroups with closely matched keywords. Yes, I don't use 1 word kw at all unless its a brand name and is allowed. 2) Start with google recommended placements first before moving to your own placement. I the later being more expensive than the former. And yes having too many negatives will just reduce or kill your ads from running.