I too like the others have no idea where you are in the recording process.
Assuming you can not get audio at all as opposed to losing audio in the middle of a session.... couple of thoughts....
First place I would look after checking what tecknot said would be to go to your audio input outputs under preferences in the menu and make sure Sonar sees your audio interface. You can access your interface with the dropdown menu. I use an Audient iD14 but I choose ASIO in the dropdown and my interface pops up. Make sure the correct channel input and outputs are checked. Normally when your Daw sees your audio interface it will automatically assign all input/output channels available with your interface. Then make sure the bit rates on your interface and Sonar reflects the same like 16 bit 24bit 32, 44, 88,96 whatever you run make sure they match.
Make sure the input on each individual track you are working with is properly selected within the input section of each track you create within your DAW. For instance...my audio interface has 2 channels. On each new track I make I can choose to record in Stereo 1/2 , or Mono1 or mono2.
If my microphone for instance is plugged into channel 2 of my audio interface and I choose audio 1 in the input section of the track I'm working with in my DAW as my input I will get no sound. Input source and selected channel in DAW must match.
Finally, make sure the output of each track is routed to the same output as your main fader. Usually Main out or perhaps in my case Audient 1&2 out. You'll find it.
Make sure your input levels from your audio interface comes in at about 18-15 on your faders. No more than 12 or you could get digital clipping in later stages of the process ...not good.
Forgive me if I have rehearsed old news to you but given your question I thought this might be useful to you.
Regards,
Tim