You say "saved projects don't play back", does it mean you can create a new project and work normally until you save/exit/restart?
Can you play back audio from other sources than SONAR using Komplete?
Did you change anything when the problem started?
Are you using both audio and MIDI?
What means "the normal stuff" you've done? Checking that SONAR and interface are set to use same (up to date)driver/sample rate?
If you use your integrated sound circuit, does anything work then =Is the audio interface maybe
simply broken?