First, welcome to WW! ;)
And second: are you sure that the site has gone? There are a lot of reasons why a site may disappear from the SERPs for a keyphrase. To be sure if your site is still in google's index, simply type your url in the search box with the
http:// included. If your page is not listed there, then it's out. If it is listed, you simply have lost a lot of ranking for your keyphrase.
What to do If your site has gone, it is possible that the cause is the inclusion of the new site (that might seem a duplicate). In this case, check for both sites to know if any of them is indexed, work to make their content different (if they have to have similar content, try to say the same with different words) and submit the site/s that is/are not indexed to G again.
If your site has not gone from the index, but from the SERPs, you should take a look to the total of files found for that keyphrase. If it's 2 million or more, you're probably a new victim of Florida. If that's your case, the best you can do is to target alternative keywords, making sure that the words are less competitive (make a search for the words and check the total of files: try to use words that give less than 500.000 results).
It's also possible that your site has been punished: in this case, you will still be in G's index, but won't appear in any search. A quick way to check that is to look your PR at the toolbar: if it has gone to 0 (white bar), it's very probable that you have been punished (unless you have lost ALL your links, of course).
If the bar is grey, it can happen for two reasons: the first is that the toolbar has not been able to connect with google, so it can not retrieve your PR. The other one is that the page is not in G's index (another way to check it), so no PR value is assigned to it. This do not happen often, 'cause the toolbar tries to give PR aproximations when no real value is available.
Hoping this is useful,
Herenvardö
