A significant number of single-member and multi-member LLCs are S corporations, and many LLCs ought to be S corporations in order to maximize their IRC section 199A pass-through deductions.  The post under the link below provides a brief but useful discussion of what an entity that ought to be an S corporation can do if it inadvertently allows its S election to terminate.

Here’s the link:  http://taxblog.dickinson-wright.com/2019/07/remedying-the-inadvertent-termination-of-an-s-corporation-election/#page=1