There are some common issues that occasionally happen when trying to run Krita. The most common issue has to do with the canvas. The pen pressure might not work, or the canvas might appear black. A common solution is to disable OpenGL. This is done from the main menu Settings > Configure Krita > Display tab. To disable it, uncheck the OpenGL checkbox. OpenGL is a graphics technology that Krita uses. OpenGL is used for canvas rotation and zooming functions. This problem is more common with older computers. Updating your graphics driver fixes this issue many times.
Another common problem is missing user interface elements. If the interface appears to be missing elements, it is easiest to reset the workspace. The icon on the far right of the toolbar allows you to change workspaces.