You can test this by only pressing “volume-up”. (Don’t press “power”!). If the phone goes to the reboot options screen on its own, you know that the power button is jammed and needs to be replaced. (Reference and more detail here: How to bring my FP1 back to life? (Power button stuck) - #14 by Stefan)
And some advice on replacement parts: FP1: Power button does not work anymore - #19 by dbrgn