Recently, a relative of mine asked me to help him migrate from an older Samsung SGH X210 mobile phone to a new dual SIM phone (also Samsung). One of the things I had to do was move all personal data, including contacts, and delete them from the older phone (before selling).
Obviously, this was not an advanced user, so he had never changed any passwords. Upon attempting to delete contacts in one go, I encountered a password request. After searching around for a bit, I found that the default password is 00000000 (eight 0s).
This password also works on several other Samsung phones – for example, the E900.
If you have not changed the default password to delete contacts (not sure if that password is used for any other functionality), then chances are, that this password should work for your Samsung phone.
Do let us know in comments if it worked for you, and if so, which model.