Hi! Can you provide a bit more information? Do you mean that the indirect effect or the direct effect of the mediator on the DV was not significant? And is the moderator added to the b' pathway?

Yes it is possible that we have insignificant mediation effect while the moderated mediation is significant . In order to understand this mechanism first we go for the justification of the moderation, we say that we can use any variable as moderator when the existing relationship is inconsistent in the absence of moderator. In this particular case existing relationship is insignificant but at high values of moderator it becomes significant. Therefore there is no problem in this case to have insignificant mediation with significant moderated mediation