If you or a colleague has moved sites, the access permissions will need to be changed so they can log in to the right location or site.
Only Admins & Managers can edit user details - if you don't know who that is just drop Trail support a note.
To change a user's access settings, edit them from the Users page.
Under Responsibilities you'll find options to assign them to a site or group/area.
If it's a different person (eg. a new member of staff) it's better to create a new account for them.
More on Roles & Permissions.