This page has a wide variety of topics. The table of contents is right below, so you can try to find your issue in there.
These tutorials assume that you have custom firmware and GM9 installed.
If you are experiencing this issue on an original Nintendo 3DS (start and select buttons are below the bottom screen), the fix requires you to remove the battery, and you will need a screwdriver.
Power off the system by holding the power button for about 6 seconds. Stop holding the power button and wait for the lights to fade.
Once the power light is off, flip the system over. Grab a small Phillips screwdriver (+ shape, PH000 preferably) and loosen all 4 screws. They will not completely unscrew.
Gently remove the bottom plate. Start from the back of the system, near the card slot and power port.
Once you have located the battery, gently remove it using the tab shown in the image. Wait a few seconds.
Place the battery back into the system.
Gently place the bottom plate back on. Be careful about the two tabs on it. Use the screwdriver and secure the screws.
When you remove the battery, it also resets the system clock, so for setting the clock, go to this page.