The FSX logbook file can get corrupted quite easily. If you don't mind losing all of your logs, you can delete the file \Documents and Settings\userid\My Documents\Flight Simulator X Files\Logbook.BIN. FSX will create a new one the next time it starts.
I hate having to restart my logbook. There is an excellent logbook editor available at
http://lc0277.nerim.net/wiki/index.php?software It is the third item on the page. I have used this program quite a bit over the past couple of years and it always fixes any problems with my log files. It is written in Java, so you will need to have Java installed on your computer. As a bonus, it is written in French.

Most likely the last entry in your logbook is corrupted. Delete it, save your logbook, and restart FSX. Fly a quick flight and check your logs. Things should be working again.