Your files are locked with a password. That password is hashed for example apple = $1$u6LCexMv$BXha6te1tEk3dBMqjju6F0 it is the hash that you files are really locked with when you change your password your new password gets a new hash for example bunny = $1$XVqEBQYz$1GETn25VbpjZGtmJfTmGk1
if you change your password back to the oreginal it may or may not work depending on what hashing algorithm you compuer decides to use.
Basiclly you are screwed, I suggest you get a computer forensics expert to help you but their services are not cheap.