My sites dropped off Google recently too...around 14th/15th of March. Went from 1st page in many terms to 5th page, or off the charts. Hasn't sprung back yet... Destroyed my traffic. Was getting 250-300 uniques on my big site, now 30-40 Hope it fixes soon.
I'm experiencing the same, I've gone from the number one result for my keywords to nowhere to be seen. I hope it's temporary, it's one thing to be ranked badly but to have a great rank then completely dissapear
my sites seem to drop off google's planet every day then come back, then go again. it's like the tide.
changing rankings are normal, check site:yourdomain.com to make sure you haven't been banned or penalized...your site will NOT completely disappear unless you have been penalized.
2 of my sites dropped 7 places and then totally gone. I have managed to get one of them back on the next crawl but i don't know what has happened to the other 1.