Your subject line and message say two different things. You mean, how do elements reserved by another user get released if his/her computer h is lost/stolen, the user not responding / fell off the grid, etc?
There is a command to force the problematic user out of the project. All of their elements will be released. If the computer is 'found', or the user decides to answer the phone and be active again, or whatever - they will have to user copy/paste or merge (after reserving again) to bring their elements back into the project.
http://www.archicadwiki.com/Teamwork/Wo ... is_user.3F
In case that full link doesn't work, here's the one to the full article: