I bought a Yongnuo YN560II external flash for my Nikon D3100 right before Christmas. When I first started using it, the shutter speed automatically adjusted as though there were no flash, but the flash fired when I took a picture. Now when I try to use it the flash won't fire at all. I have tried it in Manual mode, Auto, and Shutter Priority without luck. I've adjusted everything I can think of. Is there something I'm missing?
Make sure that the flash is not in a Slave mode (S1 or S2). If S1 or S2 are engaged, then the YN-560II stops "listening" to the pin on the foot, and only "listens" to the front red sensor panel. This is the most likely cause.
You can also check the batteries, that the pin on the foot and the contact on the hotshoe are clean and making contact (i.e., that the flash is seated properly), and also try testing the flash while off the camera with the test button to see if it's something internal in the flash vs. the flash-to-camera communication on the hotshoe.
Also, the settings on the camera are completely separate from the settings on the flash. The YN-560 is a manual-only flash. To test flash settings, you have to make them on the flash itself.
The Yongnuo YN560 II (as well as I and III) are all fully manual flashes. They do not have compatibility with automatic flash modes such as TTL or other manufacturer-specific auto flash modes.
Put your camera into Manual mode, and turn the flash on. Start with a test fire of the flash by hitting the big LED light on the flash after it (usually) turns green. Did you get a test flash? If so then your flash is working, if not then check your batteries or return the unit because it is defective. Moving on, as long as the flash is ready and in the standard manual mode, it will be triggered each time the shutter is released on your camera in Manual mode. If that is not the case it is likely a defective unit.
You can also test the flash on any camera with a hotshoe mount, including film SLRs for example. Anything that will send a signal via the hotshoe should trigger the flash as it is manual.