So I finished a large model in model builder, and when I tried to run the model, arcgis crashed. After rebooting arc, the model now has a red x next to it, and when I open it up in model builder, I can't edit anything. It will neither run nor validate. Furthermore, when I open up the model for the first time, the model builder page is blank. Only after hitting "undo" multiple times, does the model re-appear.