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Unfortunately the 2 long, followed by 5 short blink pattern directly indicates "panel power supply" which means that either the power supply board, or the T-con (the panel supply) has failed and requires repairs made to the set in most cases.
However it's worth it to try a hard reset, as sometimes the processor in the TV can simply have locked up.
The first thing you would try, is disconnect the set from power, then press and hold the power button on the TV itself (not the remote), for 10 seconds and release. Then plug the set back in, and also check that the power cord is connected tight at the TV as well. Press power once, and see if the set powers up normally. Try this procedure up to 2 times, and please let me know what happens.
No, the Mainboard doesn't give that error code, as I stated above the issue is the T-con (the panel supply) or the power supply board. The only way to narrow it down further from there is to check the 12 volt line on the power supply board that FEEDS the t-con to see if it is present or faulty.
You're welcome. If that does not cure the issue then you need to purchase the T-con (LCD timing controller) board which is located top dead center of the panel of the set- and connects via two LVDS cables to the mainboard. Let me know if you need assistance locating that part.
Let me know if you need further assistance. Also, before you go, could you please click a POSITIVE RATING (by scrolling all of the way to the top of the page, and clicking 3-5 stars), so that I am credited for working with you today?