I’ve had this annoying issue on (only one of) my Windows 8 machines, and that is the hot corners on the left side were not working. Is that what you call them, ‘hot corners’?? Anyway…
I couldn’t get the open apps list from the top left, and I couldn’t even get the ‘start button’ in the bottom left corner (although I could at least put the cursor down there and still get to the start screen – some – of the time.)
Funny thing was, the hot corners on the right side of the screen were working quite reliably!
Finally found a solution. Turns out in PC Settings > General, (Win key + W, then type: gen in the search box) you have two settings under App Switching. Both were set to ON (as they should be…) Not sure which setting did the trick for me, but I set them both to OFF and then back ON again. Wow, problem solved.