Carole, this is a new problem that started around 19 Sept. 2015.
It is apparently not a problem with Max OS, but a problem with the Yahoo server no longer supplying the data. Perhaps Apple had a fall-out with Yahoo..... who knows. No idea whether anyone is working to solve the issue or not. This is a "built-in" dashboard widget supplied with the OSX. So Apple is in some way responsible for getting it to work. Really too bad because it works/worked very well with reliable data and good graphic design; was better than most other such weather widgets.
Widgets which have not been erased still work, apparently! But if you erase it and place a new one on the dashboard, then it will show a default location such as Cupertino. IF you still have a working widget on the dashboard, DO NOT erase it. (You won't get it back.) In case you are in the input mode on the reverse side trying to inout another city and get the customary error, then the way to recover the previous city setting is to do "cmd R".
I just hope that Apple gets on this and communicates what is going on.