Sussed it, thanks.  The "Show worker data" box had somehow become unticked (why? who knows?)
If this happens, then after the next data refresh and until the program is restarted, the workers are still displayed, but the hash rate changes to ZERO.
On closing and restarting, the worker page is then blank, and the hash rate is ZERO.
If the box is then ticked, the workers and hash rate are displayed after the next data refresh, even without a restart.

Perhaps this needs a tweak, as I'm sure the hash rate should continue to be displayed even if the actual workers are not, and the behaviour when the box is ticked/unticked is inconsistent.

I'm not seeing the error which testicus has posted, though.

A