My guess is that your low limit screw was incorrectly adjusted and the downshift threw the derailleur into the spokes. Carefully check the spokes for damage that may mess up your day later on. Replace with similar derailleur and make sure limits are set correctly. But, yes, your chain may also be too short to make the 46 tooth chainring work properly with your low rear cog, and that might have snapped the derailleur as well. Switching to an IGH would be a much more expensive solution.