First try to rebuild the model. You may need to press the pause button. In some cases, you might be able to undo some parameter changes via Control-Z and then rebuild. If this doesn’t work, then one recovery technique is to build a new shoe last using the failed last as a template for the new build, making sure to set the primary measurements to the desired values.
Another thing to check is the viewport name. The perspective view gets renamed to the view currently being edited, and some errors might prevent this from returning to perspective. If this happens, try manually renaming the view back to perspective (right click the view name and set view to perspective).
In general, it is important to save regularly, possibly using the incremental save feature in Rhino.