I opened my Kindle Paperwhite the other day and there was a battery symbol on the screen. I plugged it in to charge, but this didn't help. I tried holding down the button for 7-15 seconds for a reboot. This did not help. I tried holding down the button for 50 seconds for a hard reboot. I tried this with the device plugged in and unplugged. Neither helped. Now the screen is grey with the battery symbol. I'm not sure what to do now.
Did you plug it in and leave it? If it is dead enough to give you that symbol it could take a few minutes to actually start charging and reboot. If that doesn't work, try plugging it into a computer and leave it for several hours.
Does the charging light come on when you plug it in? If so, leave it plugged in for at least a few hours, or until the light turns green. If the light is not coming on or not staying on, try using a different charger and cable.
If charging doesn't work, plugging into a usb port on computer overnight can often revive a seemingly-dead Kindle. It may have something to do with switching between charging mode and usb mode.
If it still doesn't respond after charging, or after connecting to a computer, unplug it and hold the power button in for a full 30 seconds (time it).
it actually sounds like you charging cord/cable might be bad. Did the light turn green/ if it didn't try another cord/cable. Trying to restart it by holding in the power button won't do anything if your batter is dead - it needs battery power to reboot.