Strange problem !

I've got a really strange thing happening with my DAW (Sonar X3D).  Yesterday everything was working fine...when I came in this morning and opened up a project I'm working on....I have NO audio.  Nothing showing up in the Master bus....nothing showing up in ANY audio tracks.  Even my Superior Drummer has no audio showing on the track meters.  The midi track shows activity...but no audio ANYWHERE.  My Performance meter also show absolutely nothing...which it usually shows core 1 with a little more activity than the rest of the cores.  But now it shows nothing.  I have audio tracks recorded...which show the waveform on each track...but no audio and no meter activity.  The only meters showing anything are the MIDI meters...and the audio associated with the MIDI track shows nothing.
 
This really has me puzzled...since, like I said, everything was working fine yesterday.
 
I thought I saw a thread a little while back about someone experiencing the same thing...but I searched for it and can't find that thread.

Never mind.  WHEW!  I unchecked my Focusrite Scarlette 18i20 and checked my Echo Mia for the sound card.  Same thing with the Echo....no audio.  But I then switched back to the Focusrite and now it's back to working again.
 
I noticed too that when it wasn't working...the audio engine light was off...and could not be turned back on.  Now that it's working again...the audio engine light is on.