Were you saving across a network and locally?
Are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by keeping command-alt-shift/ctrl-alt-shift pressed while starting Photoshop after making sure all customized presets like Actions, Patterns, Brushes etc. have been saved and making a note of the Preferences you’ve changed, 3rd party plug-ins deactivation, system maintenance, cleaning caches, font validation, etc.)?
I was saving to a Promise Pegasus connected directly to a Mac mini.
As I didn't lose any work, I think I'll carry on and see if it happens again, before going through all those trouble-shooting routines. But it's a recent, clean, up-to-date Photoshop install, with no plug-ins etc.
(I think I'll Force Quit now, as Phtoshop won't allow me to quit as it thinks it's saving.)