Thank you, Michael Wilde. The last paragraph with its warning is well done. I second your warning about opening and working on the inside circuitry of electronic flash units. My sense of authority to speak on this comes from being an ET who does dabble in restoring multivibrator circuits to normal function to make electronic flash units operate again.
If you are not knowledgeable and somewhat experienced in working on high voltage circuitry, DON'T. The voltages inside even small battery operated electronic flash units can be several hundred volts. If you get across it, you will feel it. You will probably react to it. Even if the electrical shock does not harm you, the involuntary muscle contractions you will experience may knock you down and can throw you across the room. This can lead to injuries from simple mechanical causes.
If you have ever turned off your electronic flash unit, and watched it, you may have noticed that the little "flash ready" light will continue to glow for several minutes afterward. The capacitors inside will continue to hold a charge for a fairly long time. Just turning the unit off will not make it safe immediately.
If your electronic flash unit does not work, and checking the fuses or replacing the batteries and checking the contacts and cables does not restore it to a working condition, consider taking it to a shop for repair. I really do not want to see you with your arm in a sling at the next photographer's gathering, or a photograph of you in a pine box.