Lilian:
There are two things to look at:
1. The User Origin is not aligned with the Project Origin. Open the Coordinates Palette and double-click the Set User Origin button (left-most button) to reset the User Origin to the Project Origin. (There is probably some other way to activate this command, this is just the way I do it.)
2. You have switched to the Rotated Grid option, which can be set with an offset but 0˚ rotation under
Grids & Backgrounds.... Switch back to the Orthogonal Grid option.
David