Mysteriously changed Security Code for 6300?

Hi, I just got my 6300 and been using it for a week or two. I put a security code for my privacy and it worked perfectly alright until today, something unusual happened. I left my phone for a while and when I came back, I entered my security code and the phone won't just accept it. I tried it million times and still won't work. Im wondering whether we can reset the entire settings of the phone, but I dont know how. Please help me. I dont know what to do.