I have a Nikon D5200 and Metz 15 MS-1 wireless digital macro flash. It used to work perfectly, but now my camera can't detect it :( Any suggestions/solutions?
Test fire the flash to make sure that it's able to fire at all.
Replace the batteries in the flash with fresh ones.
Check both the hot shoe and the foot of the flash for dirt or corrosion. Even if you find none, you can rub the contacts with a pencil eraser to clean them a bit.
Check the fit: is the flash fully seated in the hot shoe? Is there anything that could prevent proper contact? When you lock the flash connector on the shoe, does it lock smoothly and seem secure?
Check camera settings to make sure you haven't made a change that could disable the flash.
Confirm that the flash works by testing it on a different camera.
Confirm that the camera works by testing it with a different flash.
Update: Having looked more closely at the flash in question, it's apparently not a hot shoe flash. Here are a few more things to check that are more specific to that flash:
Check the IR clip: if you're using the IR clip over the built-in flash to trigger the ring flash, make sure that the flash's IR sensor isn't covered and that there's a clear path from the clip to the sensor.
Check the sync cord: as with the hot shoe advice above, check for dirt or corrosion on the ends of the sync cable and in the terminal it connects to. Test the sync cord with a different flash to make sure that it works, or try a different cord. Sometimes the wires inside a cable can break.
Update the flash's firmware: it's hard to see why existing firmware would prevent the flash from working at all, but it wouldn't hurt to make sure it's up to date.