Nikon sb-600 Flash not working

Could you provide some details? How does it not work? Does the D90 not turn on? Does it take a picture but the flash doesn't go off? What exposure mode(s) are you using?

have you tried resetting to default settings?

No--SB-400, SB-600, SB-800, SB-700 or SB-900.

perhaps it's sensing that the diffuser is out. because if the diffuser is out, the flash will not zoom (at least it was like this with the SB-80DX)

From the Nikon site, do you have one of these flash units.
Support for the Nikon Creative Lighting System when using SB-900, SB-600, or SB-400 Speedlights, or the SU-800 Wireless Speedlight Commander
If so You set the flash mode to "comander" the pop up flash must be UP

This Video by Nikon shows how but you will have to read the manual to get a full description go to the flash section http://www.nikondigitutor.com/eng/d300s/index.html

Then look at them all :)

It maybe because it overheated he flash tube, nikon flash guns use a thermal cutout, to prevent overheating, usually after approximately 30 / 40 shots, in quick succession.  It should start working again after it cools down.
If not you may need to get it checked at Nikon Service Centre. Cheers Ian

It will work great!

Definately a settings problem. The camera will only syncronize on one specific speed. Youe best bet is to use auto. For manual, you'll have to set the shutter speed at the sync speed.

Flash Sync Speed

up to 1/250 X-Sync Speed

up to 1/8000 FP High Speed Sync

Red-eye reduction
Red-eye reduction with slow sync
Rear-curtain sync
Slow sync
Front-curtain sync (normal) Flash Sync Modes

1) i-TTL: TTL flash control by 1,005-pixel RGB sensor, built-in flash, SB-800, SB-600, SB-400: i-TTL balanced fill-flash and standard i-TTL flash
2) AA (Auto Aperture-type) flash: Available with SB-800 used with CPU lens
3) Non-TTL Auto: Available with Speedlights such as SB-800, 28, 27, and 22S
4) Range-priority manual flash; available with SB-800 Flash Control

zoom the flash all the way in so you can see if the flash bulb is broke

WHICH FLASH I CAN USE FOR MY NIKON D60