Lack of Maintenance:
Exchange databases tend to be subject to corruption. Many times, this corruption starts as a minor database glitch. If the databases are not properly maintained then as time goes on the problem escalates. When this happens, you may not even notice a problem until you stop and try to restart the Exchange services, only to find that they won't start. The scary thing about a situation such as this is that it often means that you've been backing up corrupt data for an extended period of time, and your backups may only contain corrupted data. Read more on the below source URL..

Source:www.lucid8.com