Clipping mask is not working. None of the standard fixes are the problem.

Emily,

Given your description of the issue, especially that "This problem began happening yesterday",

it looks like some kind of corruption. There are different kinds, some temporary.

In the hope of temporary ones, you can try to close down and reopen the application, then the application and the computer (on rare occasions in the past it has been several times but hardly now), unless you have done those things already.

One is a corruption of a document, hence some of the questions, and that is ruled out.

Another one is preference corrupttion; you can reset the preferences as described in the following link.

Crucial: If you have customized settings that you wish to keep, first create a backup of these setting as described.

https://helpx.adobe.com/illustrator/kb/troubleshoot-preference-issues.html

Emily,

Just to make sure: have you selected both the Clipping Path (Mask) and the artwork to be clipped, in other words the whole Clipping Set?

And also, is the Cipping Path (Mask) a single (simple or Compound) path or live Type, rather than a Group?

Emily,

Is it happening with a new piece of artwork (just a rectangle or whatever) and a new clipping mask on top in the same document, or failing that in a new document?

Which standard fixes are you referring to?