Because your power on/off button still works, but the rest of your keyboard doesn't, I'm inclined to believe there is an issue in your trackpad(which has the logic board that runs the keyboard) or keyboard ribbon cable. Thankfully, these aren't super expensive fixes.
Connecting an external keyboard may give you the change to run an Internet Diagnostic. If you want to Option Boot, I believe on a Windows keyboard the "Alt" key is the Mac "Option" key.
Turn off your computer, then press and hold the "D" key while re-booting. If your MacBook is reporting a problem, you'll find it that way. You'll receive a reference code which you can lookup on Apple's website. However, I have found that many hardware issues are not reported.
Unfortunately, I think you may need to take a trip to an Apple Store or Apple repair shop.