How to fix position of object randomly out of place?

I’m following the blenderguru chair tutorial, and I just opened up blender and found the seat of the chair randomly out of place from where it was when I saved it. If I try to move it, it affects the chair leg’s position because it’s mirroring it. What could’ve caused this to happen?
Here is a blend file if you need it https://drive.google.com/file/d/1VjLnXYYnmrCqNqAjYIirJNVq9tJC7-GL/view?usp=sharing