Hi,
Sounds like your .pyi model may have become corrupted. If you also have a .mdl model for the same cube, you may be able to open this up and replace the .pyi file (File...Save As....and choose .pyi file type).
If the .mdl file also will not open, try opening it in a text editor - you may be able to recover the model by removing some sections from the file (no guarantees though)...Worst case is that you may have to rebuild the model (or dig out a backup tape!)
There should be some information on the cognos web site regarding this - from what I have seen, this is quite common problem when using .pyi files.....(thats why I use mdl files!)
Hope this helps
J