Disconnected power button cables

Solder them back on.

Grab/buy/acquire/steal a soldering iron and solder, then connect the wires to the metal plates and melt the solder over them using the soldering iron. Make sure that its strong enough to sustain a good tug, then do the other.
The polarity doesn't matter as its just a switch (unless there is an LED in the button or something, then you should test which way works before soldering).