In such cases project manager can talk to the department head and ask for a other person from that department to get to Know about the project and get started in place of the person who had his issue. This things are really out of hand where you can ask your employees to give a notice before going away from work for any sudden personal problem or any if he gets ill. So if you have matrix structured organisation you can do this. Project manager can later have meeting with this new guy and let him know the status of current project and show him from where he has to start.But if employee is out or sick only for few days than that delay is usually avoided by PM just by having that time buffer after making WBS while making the timeline.