I’m frustrated

how to set-up logic pro x & osx el capitan version 10.11.3 for m-audio25 to make sound? it used to work but it doesn't anymore.

m-audio oxygen25 via usb to macbk pro el capitan osx 10.11.3 utilizing logic pro x. there's no sound. it did work when last used, now it does not. can't figure it out.
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