Unless you make a special election to be taxed as an S Corp., a single member LLC is considered a disregarded entity, and it is reported on schedule C as part of your personal tax return. You will have to go back and file an amended return that includes a schedule C to report all of your income and business expenses, calculate the income tax and self-employment tax, and probably pay a fair amount back to the IRS unless you made estimated payments that you did not put on your tax return originally.
(Note that if you elect to be taxed as an S Corp., that only changes the tax return you file, it does not actually make you an S Corp. Corporate tax returns were due March 15, not April 15, did not have an extension due to the coronavirus, and carry a late fee of $195 per month per member.)
