Cannot re-flash beaglebone black wireless via SD card

I had a running BBBW and all was well. The unit was running the latest January image and had been configured and working on wifi nicely. Now the unit boots to a host prompt of root@beaglebone:/home/debian# and there are no directories except bin.

root@beaglebone:/# ls -a
bash: /bin/ls: No such file or directory
root@beaglebone:/# cd /etc
root@beaglebone:/etc# pwd

I cannot access /opt

I have tried to boot from SD card using the usual method without success. Any help restoring this board is very welcome.