First I would try to reactivate the mirror, if that fails or isn't available I would backup the data you have, break the raid and then immediately recreate it. I can't recall if win8 needs the mirror drive to be empty first or not but thats easy to do if its needs it.
BTW you should always be making backups of stuff you don't want to lose esp when you are about to mess with them. A mirrored drive is not a backup.
To reactivate the drive, in disk management right click on the drive that says missing (or offline) and then left click on reactivate.