Shoutbox

Never heared nor experienced it.

Actually Messenger Plus! never removes any log files when you view them. Even corrupt files, or non-log files, etc wont be removed. At best, they are simply ignored when you use the Log Viewer, but they would still exist physically on your hard disk.

So, the only reason why a log file would be "removed" by Messenger Plus! is when logs are archived to a subdirectory. This depends on your log settings* of course. But this isn't "removing" the file though; the file still exists, it is simply moved to a subdirectory...

So, all I can suggest is to check if the log hasn't been moved to an archived map.
Other than that, if you can't recover the file using a recovery program, you have lost it permanently.

*eg: In Plus! > Options & Preferences > Conversations > Chat Logging
When you have selected 'one folder per month' then logs from the previous month will be moved to a subdirectory with the name of that month.

Questions:
1) While you were viewing the logs with the Log Viewer, did you receive any messages from one of your contacts?
2) If so, was one of those messages coming from the contact from which you 'lost' the log file?
3) Do you have the following registry setting, and if so what is the exact value:
    HKEY_CURRENT_USER\Software\Patchou\Messenger Plus! Live\your@email.com\LogsDateDirFormat