Can that character talk to NPCs or other party members? A couple people have run into problems where certain characters couldn't talk and wouldn't enter combat.
Please email email@example.com, with a description of the problem and the report.zip file generated by the D:OS support tool
(for the PC version). The report will contain your saves, as well as system info, etc, which should help Larian identify the problem. If file size is an issue, older saves or those in other profiles can be deleted (if available, the most recent working save and the oldest broken save would be most relevant, as well as the most recent save).
From Bugged companion
Update: I believe we fixed it! Although the character couldn't talk to NPCs, we managed to get it talking to the Death Shrine in Luculla. This seems to have kicked it back into action, the character can enter combat again, too!
So try finding an object that you use with the cogs icon that starts dialogue, see if that works.