Do you have the compiled version of the source code (you said you lost the source code, but not the compiled program)? If you have the compiled program, you can disassemble it, assuming the password is stored in a string in the source code. If it is a .Net program (C#, VB.Net, etc.), you can use Reflector to disassemble it for you. Once you disassemble it, you will be able to view the source code, including the connection string that presumably contains the password. If that doesn't work out for some reason, there are other less appealing options.
I'm not sure if there is any tool out there that can recover SQLCE database passwords for you, but you can always try a brute force technique. I imagine connecting to the database produces an error when you provide the incorrect password? If so, write a program that continuously attempts connecting to the database with computer generated passwords until the connection eventually succeeds.
Depending on the strength of your password, this could take quite some time (if a really strong password, could take longer than your lifetime). If you can reduce the problem set somewhat by remembering aspects of your password, that should help cut down on the time. For example, if you know it was exactly 8 letters long or at least 4 letters long, you can either only check 8 letter combinations or start at passwords that are 4 letters long. And if you know it only contains numeric digits, then you only need to search using strings composed of numeric digits.
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