Robert_Charlton - 9:56 pm on May 21, 2013 (gmt 0)
jackfro - Just guessing here... it's looking like there's a big update in progress, and that Google may well have reverted to old data and is rebuilding the index. It's likely to be a major update that might take a while to settle out.
For the time being, I would simply not jump to any conclusions. In the long run, I believe that tedster and Sand are offering excellent advice.
Hard to say what your problem is, but it might be helpful to all of us to know when your traffic started to drop. Various reports have the big Google data change happening somewhere on May 16-17, 2013, which isn't to rule out all sorts of other anomalies.