Rather than rebooting the computer, rebooting the mixer, or both, you can try changing the sample rate in the mixer and then changing it back to the setting you want. Interestingly, that seems to be causing the sound to come back. It’s not a bad idea to reboot the computer and have as few applications running as possible to prevent BSOD and other later glitches, but often that’s not enough to get the sound running, especially if the mixer has been sitting there turned on & unused for a few hours. Rather than rebooting the mixer, the sample rate change seems effective in waking up whatever is going to sleep on the mixer or Windows or both.