The Notification Server (NS) is not functioning because the hard drive is full.
Use a tool like WinDirStat to analyze the hard drive and determine what files are taking too much space. Common culprits include patch files and IIS logs.
To cleanup unused patch files see this article: http://www.symantec.com/docs/HOWTO38239
To control the size of IIS logs see this article: http://www.iis.net/learn/manage/provisioning-and-managing-iis/managing-iis-log-file-storage#00