I tried to install a forum to my my website a few weeks ago and because of my poor knowledge in working in cPanel, I erased my website's database by mistake. At that moment I had 5-700 visitors every day. It took me a few days to reconstruct my site, but since than my traffic has decreased significantly. Is it possible that google placed my site into this "sandbox"?
Before this "accident" I was on first page for a few keywords on Google Image search, but after that I'm can't find my site even on page 10. :(
How old is your site?
I doubt you have been placed back in the sandbox but have been wiped out of the SERPs. This happened because when googlebot may have tried to crawl your site, it found nothing or I may say, the content was totally different that what it had previously indexed. Hopefully you are using Google's webmaster tools, use it to the full extent. Start SEO again and yeah, ask Googe for a reconsideration, but if your content is totally new then i doubt it will help?
Hadn't you taken a database Backup?