This is a weird situation: my mouse pointer is invisible until I start typing into the Windows 10 password prompt. Then the mouse pointer reappears. It is also generally visible onscreen except if I hover over the taskbar or drag the mouse pointer over into the Start menu. It becomes invisible then! I can't see what item I'm pointing at on the Windows 10 Start menu! It seems like the menu covers up the mouse, but if I drag it back away I can see it. The mouse is also hiding behind the taskbar for some reason. Needless to say, that makes using the PC very difficult because I can't see where I am clicking (clicking actually does work, though it takes a lot of trial and error!) I've noticed the mouse pointer also disappears if I hover into the notifications area on the right or try to change the date/time on the taskbar. Interestingly, the selection highlighting that accompanies the mouse when hovering over something does occur in some cases, which helps me to track the mouse pointer position a little better sometimes. Still, I want my mouse pointer to be always visible!
It turned out that my PC's USB wireless mouse receiver had been unplugged all this time and I didn't notice because I was able to use a separate mouse via a software KVM (Microsoft's own "Mouse Without Borders" tool) to move the mouse pointer instead. I had left the PC's own hardware mouse unused for a while because I preferred the software KVM mouse. Thus I hadn't noticed the hardware mouse was unresponsive the whole time. Plugging that USB wireless mouse receiver back into the PC fixed the problem. Now the mouse pointer does not hide during login, behind the Start menu, or behind the taskbar and notification area anymore. Who would've thought? I just stumbled upon this answer because I happened to casually see the unplugged wireless receiver one day!