Six months have passed since paper has been accepted and it was not yet published - has the editor changed his mind?
It is very unlikely that the article has been "unaccepted", and as far as I know, most journals publish articles basically in order (modulo some special issues and things like that).

However most journals have backlogs, and can take a long time from submission of the final copy to getting the article to print.  I don't know how common that is in your field, but for math, in 2015, the AMS published some data on how long it takes math journals to publish an article; a quick glance shows that gaps of much longer than six months between acceptance and publication are common.