ctrl+z broke the mesh

OH! That looks bad!
Have you tried switching to object mode and then back to sculpt mode?

I  notice you Dyntopo turned off also.

Try SHIFT+CTRL+Z to un-undo.

or try File > Recover > Autosave, and look for the last autosaved file.

or File > Save As, change the file name so you can go back if needed, > Save and then File > Revert > Revert, in the pop up box.

Hi ggaryhohk ,

This is probably  too late for now, but, like Adrian said, Autosave is your best bet, when CTRL+Z messes things up!

Do not do anything else, do not wait, do not Save, immediately go to Recover Autosave...

Luckily, you are not very far into the course yet, so starting over is not a bad thing (I did the Planes of the Head parts about 6 times...).

If nothing goes wrong, Save incrementally every lesson (or chapter, if you want to live dangerously)...

Also, I noticed, that you are using Blender 3.0, which is one of the 'more dangerous versions' available (big changes happened between 2.79 and 2.80, 2.83 and 2.90, 2.93 and 3.0...those changes might break things and then a lot of problems get solved untill the next LTS version, if that makes sense) ....The LTS releases 2.83 and 2.93, or even the latest stable: 3.2.1 are a safer choice.

Yeah, your jaw does seem to be a bit wide (mine's already in a later stage and smoothed...and this was one of my first attempts, so...):

I haven't watched the HUMAN course, but I remember Kent over the years saying orthographic view is kinda deceiving, it distorts the shapes, that we should work and judge our human model in perspective view. Is that still a thing? 

Yes Omar, that is still a thing...but, in this Course he starts by building the basic proportions in Front and Side Orthographic Views...this is to be able to do this systematically and using a formulaic approach. This is not the only way to do it, but it is extremely helpful for beginners (in human head sculpting) like myself.

ggaryhohk ,

My guess is, that, when you had pulled the chin down from side view, you made the chin a bit wider, but had the Grab Brush still quite large...that is why 'your' neck is also a tad broad...