If a session becomes locked due to an ArcMap crash, the session may be unlocked using the Unlock Session task. The user must be the owner of the session to unlock it. There are several instances in which a session may NOT be unlocked using the Unlock tool:

A Session May Not Be Unlocked When...

In some cases a session may not be unlocked using the Unlock task. Occasionally, you cannot open a locked session as View Only (see section below).

A session is locked if another user is currently editing it in ArcMap. The session cannot be unlocked until the user closes it.
A session is locked when another user owns the session. The session may be unlocked by using the Change Owner tool. A user must be the current owner or an administrator to use this tool.

A Locked Session May Not Opened as View Only Because...

You may want to view a session that you do not own. There are a couple instances when this may not be possible. When a session is sent to the field, it is accompanied by an XML packet. This XML packet stores edits made in the field and must be opened in an Edit mode on the Enterprise in order to play the edits back to the geodatabase.

A session with an XML packet cannot be opened as View Only until its owner has opened it and saved the edits to the database. This is also true if your Administrator has decided to create XML packets on all designs, whether they're sent to the field or not. Once the session's owner has opened and saved it, the session can be opened as View Only by any other user on the Enterprise.
No session may be opened as View Only in the field.

Locked sessions may be unlocked in Session Manager or in ArcFM by selecting the Unlock Session task from the Available Tasks list.

If an Unlock task is not available for a node type, your administrator may need to create it using the Process Framework Administration Tool.