My LG 6100 mysteriously turned itself on after I finished a phone call the other day, despite already being on (it played the graphic and went into the locked state like it would be if I normally turned it on.) Thirty minutes later when I came back to use it, the screen was black, the phone unresponsive and apparently dead. I've tried removing re-inserting the battery to no avail, and even when I plug it into the wall charger it still doesn't work and doesn't even light up or make the sound it usually does when I plug it in. I left it plugged into the charger overnight and the keypad lit up for a little bit, and sometime when I hold down the power button the screen blinks all white for a microsecond, but that is the most I've been able to coax out of it. Is there some sort of secret hard reset for these things, or is this phone completely dead?
It sounds like the battery is dead. Maybe there's a problem with the battery, charge port, or charger. See if you can take it to a store and swap out a different battery and/or charger to rule those out.