Have you read your owners manual about using non-Nikon flashes?
The xtandard note in the Nikon manuals about 3rd party flashes:
"Use only Nikon flash units. Negative voltages or voltages over 250 V applied to the accessory shoe could not only prevent normal operation, but damage the sync circuitry of the camera or flash. Before using a Nikon flash unit not listed in this section, contact a Nikon-authorized service representative for more information."
Have you checked to see the flash does not have high voltage at the contact points?
Have you checked the polarity of the current on the contact points is in the correct polarity?
See the note at the very bottom of the following link:
Hi George, Thanks for the reply. The flash used to work with the optical slave a few years ago and then I didn't use it for a while. My problem is that now the connection to the optical slave isn't working. is it worth trying to get it to work?
I would get an estimate on the repair. But with the advances in strobe technology, I would look at the SB-900/700/800/600. The Nikon CLS, Creative Lighting System, is quite sophisticated and flexible. You might be be able to pickup a refurbished SB-900/700/800/600 at a price competitive with the repair.
If you can repair the flash yourself or get it done cheaply, it may be worth fixing. If not, it may be cheaper to replace the flash than repair it. Remember that it will only work as a remote in Manual flash mode and you will have to buy an optical trigger or RF trigger to fire it remotely. Good Luck and Enjoy your Nikons!