Chkdsk fails with error message re no "direct access" to volume

I had to force a shutdown after a freeze. I tried to run the chkdsk utility
from the command line, as administrator, with the parameters /f and /r.
Instead of the usual screen showing the progress of the disk check, I got a
screen with the following error message: "Cannot open volume for direct
access."

Everything else seems to be working except system restore. I tried that
twice, using two different restore points, and it failed each time due to an
"unspecified" error.