The answer to 'my GFCI outlet won't turn on' is that there are a few common causes, including a tripped GFCI, faulty wiring, or a malfunctioning GFCI unit. GFCI outlets are designed to trip and shut off power when they detect a ground fault, which is an unintended electrical path. If the GFCI trips, you'll need to reset it by pressing the 'reset' button on the outlet.
If resetting the GFCI doesn't work, the issue may be with the wiring or the GFCI unit itself. Checking the wiring for any loose connections or damage is a good next step. If the wiring appears fine, the GFCI outlet may need to be replaced. This is generally a DIY-friendly task, but if you're not comfortable working with electrical systems, it's best to call a licensed electrician.