Digital BW, The Print

Fellow artists and technologists,

I was working on a file for two days and after saving and reopening it, 
I found that I was unable to perform any effective operations on any 
layer of it.  In the layers palette, the selected layer "bar" was shown 
in light gray (rather than blue) and when I looked at the file mode, it 
was still in 16 bit, but the 8, 16, 32 mode selections were grayed 
out.  So, PS lets me perform operations as if they're working, but 
nothing actually happens.  Restarting PS had no effect and other files 
are still fine.  I'm baffled.  Any ideas?  Did I hit some secret key I 
don't know about?

Thanks,
Walt

P.S. The layers I was trying to work on were not locked.