PROBLEM:
Every now and then, Revit will remove everything from the list of things you can Undo.
It's called 'flushing the undo stack'.
CAUSE:
Various things trigger this scary behaviour:
Making a Workset editable WITHOUT using the Worksets dialog box
Deleting a View, from a workshared project that is not 'checked out' (editable, see first point)
Deleting a Family via the Project Browser, from a workshared project where the Family is not 'checked out' (editable, see first point)
SOLUTION:
Before deleting either:
Check out the View workset manually (using the Workset dialog)
Check out individual views or families by right clicking over their name in the Project Browser, and select Make Workset Editable.