Why is my flash not working?

My external flash for my dslr camera just all of a sudden stopped working. It's a vivitar for my nikon, and I was taking a shot of my cousin and one shot the flash didn't work. The batteries are fine I think, the flash itself is on, it just won't, well, flash. What's the problem and how can I fix it?

You say "external flash". Does that mean hot shoe or wireless? If it's hot shoe, double check the contacts. If it's wireless, if you can, try another wireless flash. If it doesn't work, you probably need to send your camera in for repair. In all other cases, replace the flash, probably with a different brand.