Sixth iteration of Apple iPhone, announced on September 12, 2012. Repair of this device is similar to the previous models, requiring screwdrivers and prying tools. Available as GSM or CDMA / 16, 32, or 64 GB / Black or White.
Here's the IFIXIT guide to remove and replace the power button: iPhone 5 Power Button Replacement. It sounds like the detent spring has lost its tension if it slides without any resistance. At this point it sounds like you'll need to replace it.