Could be a few reasons why you've dropped off. Either Google couldn't find your site when it was looking to re-index it, or you've done something to upset them..!
Be careful not to do anything that could be considered as 'black hat seo'. You'll probably benefit from reading up on 'Search Engine Optimisation'. There's a lot to read, but it's fairly simple to follow and a large degree is just common sense.
There's also a lot of differing opinions based on people's differing experience with trying to get good search results - and Google changes how things work every so often (only slight adjustments can have major effects).
You can try resubmitting your site to google -
http://www.google.co.uk/addurl/?continue=/addurl
Wait a few weeks and see what happens. You can also see what Google actually has indexed of your site by searching for:
site: example.com
in Google, where example.com is your site address. Just make sure that you're putting in to practice everything that is recommended and make sure not to overdo it. The only sure-fire way you're really going to get to the top is with luck, links in to your site from good websites and putting in the best SEO practices. Search for 'SEO' or 'SEO Tips' on Google - that should help you vastly.
