Google did this a few months ago and my pet theory is they are again playing with search algorithms…which from their point of view is fine…trying to refine searching..but does play havoc with visitors and hearts of webmasters…waking up to find you are not even listed!
Hopefully it will settle quicker than last time and my only advice is to not to try to adjust pages for google SEO unless you want 1 day at no.1 instead of 6 weeks at no.5 
Yes I admit it..I did it last time to a site and 1 day at no.1 is far less profitable than 6 weeks at no.5 for a good term.
My guess is (gotta check out seochat to see if it is on the right track) is that they are trying out the previous algorithm with a few refinements and will settle much quicker this time.
If I am wrong, well, will shout (buy) you all a beer!
😡 :cheers: