The Debian Installer can be used to rescue (repair) systems, for example if they fail to boot after an upgrade. Some basic information about this can be found in the Installation Guide (look for "Recovering a Broken System" in the index).
However, not all configurations are supported by default. This page can be used to document additional rescue procedures.
/Crypto - Rescuing a system set up using "Guided partitioning with encrypted LVM"