Strangely enough the power button stopped working on my Be quiet! Dark Base Pro 900 Rev.2 computer case recently. Instead of taking all the computer parts out of the case and rehoming them in a new case I elected to try out an external power supply button [1]. A bunch of options exist for external power buttons. Some people apparently like to be able to push the power button without having to reach down to the computer location. This new option worked out well for me as I just ran the external power button toward the front of the case where I can now turn the computer on and off without any problems. This oddly enough was a good solution that worked out well enough for me and saved me a lot of work to rehome the computer parts. 

Footnotes:
[1] https://www.newegg.com/p/2A6-021F-00001?Item=9SIB7VEK713319 “Desktop Computer Power Switch Power Supply Control Adapter Cable”