When I reboot my laptop, the built-in trackpad works, so I'm fairly confident that it's not a hardware problem, but when I've been using it for a while with my Bluetooth keyboard and trackpad, and then take it off its stand to use it without them, I found that the trackpad is frozen. It can't be used even to move the mouse cursor, and it can't physically click.
It's a MacBook Pro (Retina, 13-inch, Early 2015).
I really don't even know what the likely candidates are to investigate. What are possible causes of this problem?