Shoutbox

hiya,
i have a very imp chat log saved in my chat log files. i have locked my chat logs through a password. Now the problem is that i am unable to open one of the chat logs (which is v.v imp) and i m getting the message
"this file hasnot been encrypted by Messenger Plus ! (or this file is corrupted)"
I want to open this file at any cost plz if any one of u can help me out of this situation. All the other files of the same time are accessable but not this particular one ........ Please help me in solving this problem. I want to open it !!!