Windows 10 has this funny bug where sometimes for some reason your individual windows don't get focus any more. You can click at their title bar, and they'll be drawn as-if they have focus (darker shadow and foreground titlebar), but no window actually accepts or processes any user input (while updating normally in the background, and also redrawing). The only way I know how to fix this is to go into the Ctrl+Alt+Delete screen; just bringing up task manager doesn't change anything.
It sounds like you've installed some sort of app with a low level keyboard hook that's discarding the input. The login screen is in a separate desktop and not subject to such meddling. If it was an accessibility tool it might be able to hook into the login screen, I forget.
Alt+Tab (switch between open programs) - might help change the focus
Windows 10 has an advanced version of ^ as I understand, which uses the key combo: Win + Tab. The cycle of these commands can be reversed by adding "shift" to the mix (in case of many open programs / windows and you dont want to cycle through all of them)
Win+x has different, but equally useful menus for win 8 / win 10 - I was overjoyed when i first discovered this, as it retired many many other shortcuts I used over the years. (to get to device manager / control panel / event viewer)
and if all else fails: Alt+F4 - closes active (focused) program / window. If no programs or windows are in focus, then it will bring up the "shut down" dialog box, which allows for reboot / signout / shutdown / Switch user / sleep and hibernation if its enabled.
MacOS has had a similar issue as far back as 10.7. In 10.6 and before if you see a window with red, green, and yellow buttons in the corner, that window has focus, period. It was an enforced invariant.
In later versions it's pretty common to see such windows that don't actually have focus. Clicking again on the title bar usually fixes the problem but I find it very annoying. There's a race condition in the system somewhere that Apple doesn't recognize as a bug.
Could you have something running with an invisible window, e.g. keylogger? Less scrupulous friends from way back used to talk about writing such things.
The reason why this will fix it is because Ctrl+Alt+Delete has a higher level of system interrupt than alternate ways to get to task manager. Ctrl+Alt+Delete fixes a surprising number of issues by interrupting runaway issues.
My favourite W10 bug is that if I maximise an Office window and then try to close it by mashing the mouse right into the top right corner, it actually clicks past and closes whatever window is underneath.
