Can't say anything for the ATR, however, most engines are still producing some power in these cases, so reducing the PL will most likely give you finer pitch, but still positive thrust.
If the engine has failed without autofeather, then it will normally stay at pretty much the current pitch balanced by PCU oil pressure and flyweights. In the latter case PL position is irrelevant.
I would deselect autofeather if selected, reduce the power, and then carefully reapply it to see if the situation has been corrected. If not, then shutdown.