I've had this problem more than once. Now I'm really disturbed, because I should finish my project in no time. I have a project where a maple (or stm32f103c) is connected to an esp8266 and the program has been working well, but now I need to update some features. Since my last update I've been working with other boards (ie. esp32), which I install googleing something like "esp32 index json" and then editing file preferences with a corresponding web address.
But I found out that after installing esp32 I can not find maple and esp8266 in the boards manager menu so I have to install them again. But the compiler doesn't want to compile it any more. I have tried previous versions of board packages, but getting always the same compiler error. I would not want to install the IDE again (that is how I got over the problem last time)
So am I installing/updating board packages somehow wrong? Why do boards disappear from the Tools menu? My code has not changed.