The masking tools have by default moved out into a separate panel. A number of users have reported this panel going "missing" - moved outside the monitor's extent or simply not displaying, who knows. This may correlate with running LrC full-screen.
Some people report that this masking panel may come back (at least for a while) after closing and reopening LrC - and may also be better behaved if the app is not run full screen.
There is another way to show this masking panel, once you can regain sight of it - and that is, dragging to "dock" it within the right hand stack of tool panels - just under the histogram. It does seem to work reliably there.