There are two most common reasons for what you describe - back light not working or main board failure.
We've seen both in 47LV5500.
if your TV has glow coming from the panel then back light works and you should not worry about it.
if it does not then it is still not certain the problem is back light, but it is more likely; best thing to do is individually test with a LED tester device.
47LV5500 is an edge lit TV, which means the strip is at the bottom of the panel; power is sent to it via strip cables which sometimes burn and cause what you describe; other times an LED bruns resulting in open circuit and shutdown of the LED driver and again what you describe.
In both cases LED problem repair is a local matter; it is possible to do it yourself, but unless you are handy with electronics you will be better off leaving it to someone who has done it a few times before and learned to work around the risky points (and there are some!).
The second option is a bad main board. We;ve done quite a few of those and I'd say there's maybe 75% chance that's what it is, at least as of today; LEDs age as well so over time this ratio may change.
See
http://www.coppelltvrepair.com/p/1626/ebt61542604-61542604-main-board-repair-service-from-lg-47lv5400-ubausylhr-led-tv
Hope this helps!
Repair service for LG 47LV5400 UB AUSYLHR main board...
EBT61542604 61542604 main board...
Probable board issue
press link below
http://www.repairfaq.org/samnew/tvfaq/tvtmmqcs.htm
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