The Scenario:

Gymnasium, several rows of 400W MH ceiling fixtures.

One row (all on same circuit) has 10 lights fail on the same day. 2 lights on this circuit still work.

Other rows of lights on the same panel are fine. These are on different breakers.

Voltage is good on all 3 legs and at fixtures, breaker seems fine, can't find any reason for this.

Installed new ballasts and everything works.

Anybody have any guesses what happened?