As others have said, this is a "Known Plaintext Attack". All good cyphers are proof against such an attack. Any cypher which cannot withstand such an attack never gets off the starting blocks.
The best suggestion is to find out the specific encryption method used and look for specific weaknesses in that particular method. Alternatively, ask people who knew the key if they can remember even part of the original password. "It began with a D" will reduce the work you need to do to brute force it by a factor of 26 or 52.