Your cron scritps stop running.
Examine your log – most often /var/log/messages. If you find there something like this:
Sep 27 19:00:01 ns2 crond: Authentication token is no longer valid; new one required
then the password of a user which runs the cron scripts must be changed. In my case this was typical for Mandriva Linux distributions.
Change the password of a user which runs the cron scripts. This user is most often root. Use the ‘passwd’ command to change the password.