Why is my neutral bar getting red hot?
This type of situation happens when the main neutral fails and all imbalanced current is redirected to a improper/loose connected grounding electrode, and/or improperly sized grounding electrode.

As mentioned by peter and Tester101, this requires immediate attention. Your 1st move is to cut off the main power. 2nd call a professional (unless you already have the main power off and are comfortable with Harper's advice).