I was using a brush to soften skin, and my mouse suddenly stopped responding. It will not allow me to move in anything but an up/down motion.
I have changed the mouse, and I have rebooted the computer. I have no additional displays, and no other programs working.
I have noted that mouse control is an issue in more recent versions of LR as well, but I cannot locate any useful solutions. Any assistance would be greatly appreciated.
Try deleting your preference file.
Also, if you are doing lots of brushing on a single photo, you will find that Lightroom does slow down and lag, this is a known issue with Lightroom. The solutions are to do the brushing as the next-to-last editing step, and the last editing step is to turn on Lens Corrections; get a faster CPU; do the brushing in Photoshop or Photoshop Elements.