Try restarting the Leap Service to see if that resolves the issue for you. You can find instructions on how to do so here: https://support.leapmotion.com/hc/en-us/articles/223784008-How-to-Manually-Restart-Leap-Core-Services
If that doesn't resolve the issue for you, try uninstalling and reinstalling the Leap Motion software. If the issue persists, try using a different cable or USB port