It happened now a several times that I created a model in Model Builder (ArcMap 10.5.1) and when I try later to edit it, the model builder is not opening. I tried to open the affected models from ArcMap and also from ArcCatalog but without success. It might be that I had minimized the model builder window when closing the mxd file but I also closed all other windows hoping to find the small window for the model builder somewhere. No chance.
Does anybody have an idea if there is a way to recover these models somehow? I wasted already so much time on redoing models that suddenly didn't open again. 😞
Edit: I was able to open the model in ArcGIS Pro, saved it under a new name, exported the toolbox from ArcGIS Pro to 10.5 and was then able to open the model in ArcGIS again, only that I had to fix some of the processes again. I would still be interested if there is an easier way...