Whenever a document is opened from the SharePoint Document library it must be closed. When the document is opened with the Microsoft word in SharePoint document library for editing, SharePoint keeps a temporary lock on that document. Once for every 10 minutes the office product checks with the SharePoint and let it know that the document is edited by the user. Whenever the document is saved and closed then the lock is released and now the document is available for other users. When the system freezes or connection to the SharePoint environment is lost while editing the document, the lock still exists on the document. The follwoing article clearly explains this scenario in detail and the workarounds available for this issue.