Hi Kiara,
What Blender Version are you using and did you change Version in between?
Apart from the fact that the Mirror Modifier is always symmetrical, it looks like your UI is missing things:

This can be caused by opening a file made in 3.x in 4.x…or maybe your (graphics) drivers are not up to date…?
If you can upload your .blend file, that would be great (preferably with the Mirror Modifier).