id still take it to apple and see what they can do. if you can jailbreak use activator to lock, i did that when my 5 broke and double tapping the status bar ended up being much better than pressing the lock button.
Depends where you live - mine's done the same thing twice, now on my 3rd phone. Apple replaced the second one under EU consumer laws as it was out of AppleCare.
Even if you're not in the EU, worth going to Apple - you may be lucky!
I recently repaired an iPhone 5 with a similar problem that you describe. The sleep/power button was still "colicky" like normal, but could not register when I pressed it. I ended up just replacing the power/volume/silencer flex cable, and it started working again (ie, I could put the phone to sleep and power it off).
Try Apple Store first, and if they cannot help, you can try replacing the flex yourself. FYI, it was my first iPhone 5 I ever worked with, and I replaced the power flex and screen...took about 3-4 hours, but well worth it. Sent from my iPad.