Loud knocking sound, no check engine light, no acceleration. Thoughts?

Sure doesn't sound good. I have heard that Hybrids are known to be hard on the gas engines but I'm not sure of that is your problem as you stated that it returns when you turn it on so that would tell me that it is something in the electronic switching system. I hope your shop can track it down. Please let us know what happens.

PLEASE promise to come back here and tell us what the outcome of this knocking is. I'm guessing that it isn't electrical. If it's a knocking / banging that you can feel in the pedals or steering wheel, it's a bad sign that a rod in the engine is bent or broken. Losing power would be a symptom. There's a slight (very slight) chance there is a loose engine component, like the starter or alternator OR you ran over something you're dragging and it's interfering with the drivetrain somehow, but my money of on a bent rod. Sorry, I hope I'm totally wrong. Again, PLEASE follow up here with the diagnoses. It really helps others.