You need to provide more details. Registered as what? Single member LLC? Multi-member LLC? S-Corp? C-Corp? Something else? Typically (i.e.; not always) an "unregistered" business is considered a sole proprietorship. But for all we know you may have had a partnership.
So what did you register the business as?
What type of business was it before it was "registered"?
In what tax year was the business (regardless of registration status) officially open for business? In that year, did the business have any income and/or expenses?
A Form 1120 is filed with the IRS by a C corporation.
The corporation must be "formed"; articles of incorporation must be filed with the state. Further, the corporation must apply for an EIN with the IRS.
If the entity is eligible (such as an LLC), it can elect to be treated as a corporation for federal income tax purposes by filing Form 8832. An EIN is still necessary in that instance.
In short, until the IRS is aware of its existence, submitting an 1120 is useless and inappropriate.