If your corporation had first year in 2007, then Form 2553 -- election for S-corporation must be filed within two months of the beginning of the tax year. This also means that you can not even elect for S-corp for 2008. If you file now, you will be treated S-corporation from 2009.
Also instructions to Form 2553 says that if you had some reason for not filing the form as required, then you can file it with Form 1120S and attach statement. So check with IRS if you can still file Form 2553 for 2007 election (if you have some good reason) as attachment to already filed Form 1120S.
If you are a C corporation and you filed your individual tax return based on K form from S-corporation, then you will need to amend your tax return. File Form 1040X.