A loss can be carried back if it is a (NOL) Net Operating Loss (which is more than just a loss on a Sch C).
So for example if you have other income on the return, and the loss is not more than the Sch C loss, you have no NOL.
So first determine if you have a NOL.
Then it gets carried back three years (unless an election is made not to carry it back), and then it is carried forward until used up.
This is a complex area and suggest you look at following links:
https://www.irs.gov/publications/p536/ar02.html
Here is a prior answer on this topic that may be helpful.
https://ttlc.intuit.com/replies/4896446
