Often when that happens it means there is something wrong with the graphics driver. Perhaps a Windows update broke it. I guess it worked before and now it is like this without you having intentionally changed anything?
What you could also try is resetting Krita’s configuration to it’s clean default state. Here is a simple guide for that:
The reason reinstalling Krita does not solve anything most of the time is because it does not touch the config only replaces the binaries.