First: The passwords in FileMaker can not be retrieved.
There are different approaches. One is to give the customer a special superuser account that is permitted to change passwords. Not as full access, but as described here.
The superuser can log in and get access to a special layout.
On this layout there are two global fields. Account Name and Password.
Then the superuser can enter information here and can set a new password. Use the account script steps to reset the password.
If you use the option "Reset Account Password" you do of course need to remember the accountname. Any attemt to change the password of a non existing account will of course be rejected.
You could expand the model with a table with the user name (account name), thereby only having to remember/reset the password.
One extra hint: Depending on the privileges you attach to the superuser ... you will probably have to let the script run with "full access".
How secure do you need your solution?
If NSA quality security is not a big issue, you could make a guest account that has 1 Layout for reseting the password through an ecrypted string you generate. As for the 24/7 service it is not your fault they forget their passwords so if you reply in a timely fashion it should be ok. But if 24/7 service is what you are after you will need to do alot of work, as it requires a something simular to resetting your password on web sites which envolves a lot of code.
Have a look at http://www.briandunning.com/easy-encryption/ it can be modified to suit your needs.
Good Luck :-)