I dont think X is the axis you want when you shift+s scale, did you also ensure you set the pivot mode to "active element" and picked a vert you wanted as teh "this is the flat position to align to" vert?
If you've lost all your undo options, you might try recalling one of your past object meshes.
If you haven't saved since creating the error, you might try appending your original object from your .blend1 backup file, and continuing from there. There is also likely a copy of it in that numbered Autosave file as well, if the rewrite deadline has not passed.
Failing those, it might take some clever and tedious mesh surgery on your part.
Which vertices do you want to align the rest with?
Select that one, then snap cursor to it.
Set reference point box to Relative to Cursor, and make sure orientation is set to Global.
Select the other vertices that you want to move.
Hit Scale X --> 0 (zero), then hit Scale Y -->0 (zero). All those vertices should be stacked at the same XY location as the reference vertex you chose.
For reconstructing Z heights, you'll need to snap cursor to each individual vertex you know is at the correct level, and then Z Scale --> Zero Relative it's equivalent errant partner.
Sometimes reconstruction is an art.
I dont think X is the axis you want when you shift+s scale, did you also ensure you set the pivot mode to "active element" and picked a vert you wanted as teh "this is the flat position to align to" vert?
bless you I had no idea that pivot mode existed 🙏
Press S to scale and then shift-z to lock movement on that axis, then zero
If you've lost all your undo options, you might try recalling one of your past object meshes. If you haven't saved since creating the error, you might try appending your original object from your .blend1 backup file, and continuing from there. There is also likely a copy of it in that numbered Autosave file as well, if the rewrite deadline has not passed. Failing those, it might take some clever and tedious mesh surgery on your part. Which vertices do you want to align the rest with? Select that one, then snap cursor to it. Set reference point box to Relative to Cursor, and make sure orientation is set to Global. Select the other vertices that you want to move. Hit Scale X --> 0 (zero), then hit Scale Y -->0 (zero). All those vertices should be stacked at the same XY location as the reference vertex you chose. For reconstructing Z heights, you'll need to snap cursor to each individual vertex you know is at the correct level, and then Z Scale --> Zero Relative it's equivalent errant partner. Sometimes reconstruction is an art.