Unfortunately this is a
common issue that affects all applications. When a computer crashes, there is a possibility that files on your computer will be damaged in some way. This is why sometimes, after a crash, a computer will not boot up any more and may need to go through a recovery process. Even when the computer boots up again, there is still a possibility that files have been damaged or lost. Windows will normally check your disk on boot up after a crash to check for and recover damaged files but this may not always work.
If your mIRC settings are back to defaults, this means that your mirc.ini settings file was lost or damaged during the crash. If you backup your computer on a regular basis, you may be able to find your "mirc.ini" settings file in your backup and restore it. If you do not have a backup, you will need to configure your settings again.