You'll use the 2020 tax forms to report the final return for that business. If your business files as a partnership or S Corp, you'll file a 2020 tax return with a short year and indicate that the business closed on 3/31/2020.
The requirements for filing the business tax return are the same as for a return for a full tax year, except ending on the last day of the short tax year (for you 3/31/2020). For example, if you are filing a partnership Form 1065, the return is due on the 15th day of the fourth month after the end of the tax year (July 15, 2020) and if a S Corp Form 1120S, the return is due on the 15th day of the third month after the end of the tax year (June 15, 2020).
If the forms aren't available by the due dates, you have a couple of choices. You can either file an extension in the hopes the new forms (and software) will be available. Or, you can prepare the return with the "prior year" tax forms (and software) that are available, and make the appropriate adjustments and overrides. If you have to file with the prior year tax forms/software, you will need to print the return, mark out the prior year dates on the printed form, and manually write in the year for which you are filing.
Note that if this is a Schedule C business, the 2020 Form 1040 you file will be for the full year, and isn't due until 4/15/2021.
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