>>1013003
The mirror is based on the origin location of the object along the axis assigned in the modifier. If i were you here is one way how i would fix it based on the image posted.
>All operations are from front view.
1: delete mirror mod in side panel
2: object mode. select the unmirrored object, right click and set origin to geometry to reset origin location.
3:in edit mode, vertex mode, select bottom rightmost vertex and press shift+s then cursor to selected, then object mode, rightclick and origin to 3d cursor.
4: shift+s cursor to world origin
5: object mode- select object-shift+s, selection to cursor.
>your model should now sit on the axis at 0, with half on the side of centre.
6: apply mirror mod with correct axis selected in modifier panel.

There are many ways to do it, but if you are not yet used to how the origin relates to the mirror mod, or how to manipulate the origin and 3d cursor to move your objects into precision locations then those steps should help.
REMINDER
the mirror ALWAYS relates to the objects ORIGIN location (Orange dot when selected)