Search Engine Journal has a good post on What To Do When Your Established Site Drops in Rankings and gives this advice:
‘All I can say is that many have suggested the same advice in the WebmasterWorld thread. Brett Tabke, the founder of WebmasterWorld, suggested to do “NOTHING”. He said, “The worst thing you could do, would be to go make a bunch of changes. Just let the algo work itself out.” A senior member said that he “made the mistake of making many changes after taking the hit, many months later no where to be found.” ‘
I totally agree with this advice out of my own experience of disappearing from Google late last year. The temptation was to make some major changes at that time but I decided to stick it out and just keep working on some of the basics including writing good content, making the coding of my site validate, building relationships with other sites in the hope that I’d get a few extra incoming links.