What document or template are you trying to change it in?
The alignment will be set in whichever template you are using. If you are starting with a new, standard, blank page, then possibly you have altered the standard template (called normal.dot) either deliberately, or accidentally - or it has become corrupted (which dies happen).
You can delete the normal.dot template, which will force Word to create a new one, which will have left alignment set.
If you need further instructions or explanation, please just ask again.