If the locked (LKD on smaller interfaces) light on the front of your interface is off and it won't pass audio then the "Sync source" in Saffire/Scarlett MixControl is likely incorrectly set to a digital input. Open MixControl and set sync source to back to "Internal". You should also make sure you have the latest version of MixControl from the Downloads section of the Focusrite website. We, however advise you to read through the release notes before upgrading to check if it is compatible with your OS.
If you intentionally set the sync source to a digtal input such as "ADAT" or "SPDIF" and the locked light is still off, you should check that the device sending the digital signal is correctly outputting, that it's at the same sample rate as the Focusrite interface and that your connections are in order.
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