1. Make sure that the stuff needed to run CFW are still on your SD card.
2. Did you drop your DS recently? If the top screen's ribbon cable is torn, your DS won't boot properly, and you'll need to buy a replacement screen.
3. Make sure the SD card is properly inserted.
i did some sleuthing and found out that it's possible that the abxy board is damaged or the power button is dirty, for starters, the day i bought it, it took quite a bit of force to turn the console on, sometimes taking multiple presses. also if the screen ribbon cable had torn, would that affect the power button? it doesn't light up at all, it doesnt even do that faint red flicker it does when the battery is damaged, i also took a look at the battery and it looks fine and so do the contacts, some other people suggested it might be a blown fuse, but i find that hard to believe since my 3ds was turned off and not plugged in at the time when this happened, so it couldn't have been an electric overload. i did infact drop it from a distance of about 2.5 feet while it was in my backpack, though i imagine that the cloth absorbed at least a tiny bit of damage, i even checked after i dropped it and it worked just fine, a couple hours later, at night, i turned it off, took out the sd card, put it back in a minute or so later and tried to turn it on, but it wouldn't budge, so to me i think the most realistic problem is abxy board damage or lint/dirt on the power button. who knows? maybe the previous owner accidentally poured a liquid on it as well and that's why it was so hard to turn on and the cfw files are still all there.