I had the same problem once with one of my Fenix LD20's, and discovered that there was a simple fix that took maybe 10 seconds. Hope this will fix yours. Inside the tail there is a silver metal retention ring with two indentions on opposite sides. That ring can come loose, and cause the symptoms you described. Use a pair of needle-nose pliers and insert the tips of the pliers in those two indentions. Then tighten it up. Hopefully it will be good as new.
I'm an avid runner, and run before sun up. I have been looking for a long time for the perfect running light, and have come up with some lights that I consider near perfect. My favorite running lights are headlights. No more bouncing around in my hand, and no more holding something in my hand the whole time either.
My favorite light for running is the Zebralight H600. This light runs on an 18650 Li-ion rechargeable only. If you bought this light then you would need to get an 18650 (AW brand is best) Li-ion battery and a good charger (got mine over at 4sevens.) The output though is incredible (ranges from 0.1 lumen to 750 lumens,) and has the best UI around. The headband that the light comes with is too bouncy for running, but for $5 you can get (in my opinion) the best running headband around called the NiteIze headband. Got mine at Battery Junction.
If you are unfamiliar with Li-ion lights, or just don't want to go this route then you should consider my second favorite - the Zebralight H51. The H51 runs on a single AA battery (alkaline, lithium, or NiMH.) It runs best on either lithium batteries (like energizers in the blue pack at walmart) or on NiMH rechargeable batteries (like eneloops.) Output on this light ranges from 0.2 lumens to 200 lumens, and has the best UI around. You will also need to get the NiteIze headband for this light as the headband it comes with is too bouncy for running.
The NiteIze headband fits both of these lights.
Here is a thread that talks about these & other lights that are great for running -
http://www.candlepowerforums.com/vb/showthread.php?326064-Head-lamp-for-running