Dear Mr.Duval,
I am writing to inform you that I cannot take part into the seminar tomorrow morning because of an unexpected serious headache.I have not sleep well for almost a week,which takes major part of my energy away.In order to avoid from ruining the important seminar,my doctor suggested me having a rest for at least one week.I feel sorry about my absence,but that is the most I can make up.I promise that I would take a good rest and get back to work as soon as possible.
yours xx
第二个有点专业~就写一篇吧~