First of all, be sure that you don't actually have an S-corp. Did you ever file a form 2553 with the IRS electing to be taxed as an S-corp? If you did, did the IRS send you a notice that it accepted the election? If so, then you're an S-corp. If not, then depending on the facts, it's possible that the IRS will still allow you to choose to be an S-corp. Check out the instructions for form 2553 (http://www.irs.gov/pub/irs-pdf/i2553.pdf) and look at the section on P. 2 called "Relief for late elections."
Otherwise, yes, you will need to file form 1120. If you don't, then you incur penalties for not filing when due (and possibly for not paying taxes when due.)