After the mash is already **** up there's not much you can do... try deleting whatever you think is unnecessary from the hypergraph(First delete all connections among whatever mashes you find there), that might work(It did for me anyway).

about undo - if you just loaded a file and you don't have some advanced settings activated then you're not supposed to be able to undo. Another bug I noteiced is that Ctrl+Z doesn't always work, so try Edit->Undo.

Joopson is right. You can AVOID this bug by not undoing Poly->Sub-d transformation. After it happened undo is quite useless...

Hope it helps,
Benny