Hi, I have an iMac (Retina 5K, 27-inch, Late 2015) running El Capitan and I've been experiencing this already three times: the cursor gets stuck in the upper left corner of the screen and the only thing I could do was a hard reboot by pressing the off button. How can I unstuck the cursor without rebooting? How can I fix this so it won't happen again? Using another mouse doesn't help: right now connected with the iMac are an Apple Magic mouse, a USB Apple keyboard, and a USB IBM keyboard with trackpad and pointing stick, that I use just for the trackpad. None of the three methods (mouse, trackpad, stick) is able to move the cursor. The IBM keyboard works flawlessly on another iMac.