ORPHANWARE Most legacy software problems are not technical at first. They are information problems. The program runs, the business depends on it, and nobody can say who wrote it, where the source code went, or what happens the day the machine it lives on stops working. These guides answer those questions in the order owners usually ask them, starting with the definition and ending with the checklist for starting a recovery.
Each guide is written to stand on its own, so read the one that matches your situation. If two match, start with the vendor guide if your vendor has just folded, and with the recovery guide if the source code itself is the thing that is missing.