If you're sure the machine is up and that Apache and MySQL are running, it may be that one of these applications are running out of disk space.

Apache is executing PHP which is the language used by Officity in order to access the database and process the informations. PHP is using a session mechanism in order to keep you authenticated. This session mechanism uses the harddisk in order to save its datas. Be sure that there is sufficient space in order to save these sessions.

If none of your harddisks is full, continue to the next step.