I used to receive emails quite often about this issue with older versions of mIRC but hardly ever see it reported now. Newer versions of mIRC should be, and have been designed to be, more robust when saving settings. They save settings to a new file, without touching the old one, verify that the new file has been saved correctly, and only then delete the old file, so there should not be any circumstances where all settings are lost. The only time I receive emails about loss of settings nowadays is when an anti-virus application has mistakenly deleted mIRC settings and/or files due to a false positive.
In the situation described in this post, where Windows itself crashes, the crash and the side-effects prior to the crash could affect running applications in many ways, including file and memory corruption. There is nothing mIRC, or any other application, can do to prevent loss of data in this type of situation. Regarding backups, this is really something that all users should do with backup software on a regular basis to protect exactly against this type of data loss.