If you are not incorporated then just stop making quarterly estimates and stop filing a Sch C with your return.
In the program interview for the Sch C simply indicate the business was closed and then retire the assets if you have any. They were either sold, scrapped or converted to personal use ... follow the interview screens.
Assuming it was a single member LLC reported on SCH C as a part of your personal 1040 tax return, it may not be as simple as "not filing a SCH C". Expecially if the business has/had any depreciable assets or inventory.
If the business has assets, work through each individual asset one at a time and indicate that you "stopped using this asset in 2020" and then work through the screens to show your disposition of that asset. Either it was sold, lost, stolen, scrapped, removed for personal use, or whatever other reason may apply.
If the business has inventory on the last day, then that inventory needs to be disposed of somehow if you did not sell it. Typically, I see it reported as "removed for personal use" which is fine.
If the business claimed "any" business vehicle use, then you must show the disposition of the vehicle. Even if that vehicle was less tha 100% business use. I most commonly see vehicles with less than 100% business use reported as "removed for personal use".