Rumors are that Mitch McConnell, who's not been heard from in weeks, is deeply incapacitated. No idea if true.

But there is no 25th Amendment for Senators. The only way he can be replaced while living is by resignation (which requires his cooperation) or expulsion (requiring a 2/3 vote).

So if he's indeed incapacitated but living, his seat becomes effectively frozen for the rest of the term. (He has no way to resign and expulsion would require Dem cooperation).