Hardware failure, hard Systems stops. You must repair the system to bring it back online. At best you may be able to restore partial operability after removing the failed component and restarting without it. If a component is not available, but the server can still operate in a degraded manner, you may be able to reinsert it into the farm until you can get the component. Note that high end workstations could be tasked to take over this job until the parts arrive.
System stays up in a slightly degraded manner.
Software failure, soft System stops temporarily. Users must wait while Server or a service restarts. Software failure, hard System stops. System Administrator must be called on to bring system back up. Depending on damage, this may take several hours.