So many things.
- Add headphones to the mixer - does any sound come out when you're singing/talking?
- Is the device fully installed and recognized by Device manager with not error (and drivers up to date).
- Some desks 'default' to mute and you have to arm them before any sound comes out.
- Is the gain high enough. If the gain PAD is on it will reduce the gain?
- Does your microphone require phantom power?
- Are you using a jack cable or XLR? If jack, then it may 'see' (hear) it as an instrument and therefore the gain will not be high enough.
- Are you trying to hear yourself live, or will it not record you.