Hi David,
For the issue, I want to confirm whether you have an administrator account with password of this Office 365 tenant.
If so, you can reset password for the users. About how to reset passwords for users, please refer to the link below :
Reset a User's Password:
help.outlook.com/.../ms.exch.ecp.resetpassword_password.aspx
If not, you cannot use the services in this Office 365 tenant when you lost the passwords for your Office 365 accounts.
Thanks,
Linda Wang