OK here is the story,  I was at work and we have these huge mercury vapor or metal halide (not sure) light fixtures mounted on these posts about 150-200 feet high.  I accidentally bumped the light switch.  Seeing that it went black i immediately turned it back on but it took about 20 minutes before the lights came back on.  There were 6 lights on the post.  5 came on and the 6th one started smoking and then exploded in a shower of sparks,  during the explsion, many lights in the yard dimmed for a second.  MANY MANY sparks came out of it, it finally caught on fire and then went out.  what happened.

Sounds like a MH ballast failure! That fixture will probably get thrown out instead of re ballasts because its probably very charred lol

capacitor failure?
eol lamps sometimes explode but its bang and it goes out.
shorted ballast can flame out too.

did i do anything wrong by turning it off and back on?  It sizzled and crackled for about 5 minutes before the big explosion.  it was quite the fireworks show. I THINK it was MV due to the cooler color and the light style was the kind you would find in a stadium..

if it the color of light you find at the Stadium then it Metal halide. it pretty much had it ballast that fail
 that ballast just happen be on it way out.

did i do anything wrong by turning it off and back on?

This may have triggered the final collapse, but the ballast was most likely on it's way.
Normally the intermittent supply disconnection should have no adverse effects except the 15 minutes of dark time and a normal lamp wear of the extra switching cycle.
If the lamp would be the onlything to fail, there would be no smoke, nor fire, nor other fixtures dimming out, only a flash and then darkness only of that failed fixture.
The darkenning of the other lamps really mean there was a mains overlad fault, what could mean only failed ballast - the core and coil part (maybe as a result of previous capacitor failure).

It should perhaps be added that HID lamps (it applies to mercury, sodium and metal halide likewise) do not strike with hot vapor, they first need to cool down. Special hot restrike circuits, which produce strong pulses in the kilovolts range, exist for applications where this would be an issue, beyond scaring people who turned off the lights by mistake and fear they have caused damaged because just nothing happens for at least some minutes when immediately turning on the switch again.

Same story here than with an incandescent lamp that gives up its ghost when turning on the light. It would have happened anyway.