I had the same issue recently after I took an update to IOS14 - actually, I had both of the issues for the weather app reported in this thread (weather app not showing any info, or weather widget gone). Restarting my phone made the weather widget available again, but currently there are two issues: if I'm using my location, it returns no information. If I select one of my saved locations, it returns information for that location, but the info displayed in the widget is old / inaccurate (which I believe is a third unreported issue). At least selecting the widget takes me to the up to date weather information for my current location at least. A user suggested resetting my privacy and location data; I tried that, and it still displays old information on the widget, and setting the widget to location still results in no data making it to the app.