The tool wheel (and all menus) can be moved around the canvas to best fit your comfort and workflow. By default on a tablet, the tool wheel will appear as a full disc in the upper corner of the canvas. Chances are, if your tool wheel has gotten stuck in the corner or disappeared off the canvas, you've inadvertently swiped on a menu icon and minimized the interface.
If the tool wheel is stuck in the corner, tap+hold on the outer ring until it lifts from the canvas and drag it toward the center of the canvas. Alternatively, you can swipe inward on any menu icon. This will reset the position. You will find this easier to do with your finger rather than stylus.
Compact Mode - This corner position is called Compact Mode. You'll see the other menus are also minimized to give you more drawing space on the canvas. To achieve this mode, swipe outward on any menu icon or tap+hold+drag the color wheel to a corner of the canvas. It will lock into place and the tool wheel will become spinnable.
On a phone, your tool wheel will automatically be in the corner of the canvas to conserve working space.
Hidden Mode - To hide the menus completely and maximize your drawing space, swipe outward again on a menu icon. The tool wheel and menus will disappear from the canvas. Tap the tab in the upper corner to return them into Compact Mode. Swipe inward on any menu tab or tap+hold+drag on the outer ring of the tool wheel to return it to full capacity.