Check if taskbar is not out from screen.
Put mouse on bottom border, if cursor disappears totally, screen is more big that display area, try to use over and under scan settings, or correct resolution.
If still see mouse cursor tip on border, screen size are correct. And taskbar really not on screen.
Try to restart with this command (terminal or run box)
And, if problem persists, show us more details, like OS and version on use, RPi model, type of display (tv or monitor), etc.