Mathcad 15 just hanging

Hi,

I have a chunky (21 Mb) Mathcad file that has got slower and slower and now is just hanging.  It does not register as 'not responding', I can scroll, but cannot click into the screen, and the sheet is not re-calculating.  I was mid save, and had about 1 hour work that was not previously saved.  I would rather not lose it.

Any ideas?

Thanks

Frank