Sometimes, Google will place a block on searches. These occur for a variety of reasons only really and truly known to Google, but we have discovered some determining factors (such as the speed of inquiries). The good news is the blocks are always temporary.
If you suspect a “Google block” has been temporarily placed on you, go to Google’s site and attempt an “exact” (with quotes) search for one of the phrases which you are seeing this behavior.
Usually, a block with no CAPTCHA is more directly related to the search term, because normally Google will send a CAPTCHA image. There are some SOC words that Google will absolutely not return values: for example, any word that contains the word “sign” (like resign, design, etc.) or “comment” or “guest”.
Google blocks normally last from 1 to 4 hours. It’s not completely substantiated, but clearing your internet history and temporary files may speed up the process.
If you are running any other SEO-type software which uses Google data, your chances of being blocked will grow substantially.
This is a ‘temporary’ block, detailed in the link below:
http://www.google.com/support/bin/answer.py?hl=en&answer=86640
There’s no set rule on “when” a Google block will kick-in, though Google states it happens when they see a significant spike in queries. In my testing, it seems that 16 seconds between requests is the “sweet spot” for ensuring no Googleblock will occur.
If you use the Background Download Service (which does a request every 19 seconds), and someone else on your network is also doing Google queries, you may be blocked.