For security reasons, passwords can’t be retrieved.
If there are other administrators apart from you, you can write and ask them to change your password — make sure they send you your new codes. They only have to go to the "authors" section, click on your name then on “Personal Information” and choose a new password.
Another, and simpler, way to change your password is to use the "Password forgotten?" link on the login page. An e-mail message will be sent to the address with which you have registered on the site. The message will include a special link in order to confirm your identity (assuming that you are the only one who can read your e-mails) and from this link you will be able to change your password.
If you’re the only administrator you’ll have to go through the configuration process again so as to create a new access (see "Configuration Procedure" in Installing SPIP). Don’t worry, you won’t have to upload any files. It’s just a question of recreating an administrator’s access — the database won’t be affected.

SPIP 1.9.2