You might want to move so ancient project to a new location/computer just in case it is loading the toolbox from the same location, where it will obviously not work) or perhaps try renaming it, start a new project and load it and examine it. OR maybe you used it on another machine? if in a multi machine environment
For future reference for others, always save an image of your model from modelbuilder (it is one of the options)
and always export your model to a script and check/save the results from the Results window. This will allow for a manual rebuild if all is lost.