You might try a couple of things:
First, have you checked to make sure that your CAPS LOCK key is turned on in your Keyboard Preferences?
Go to System Preferences -> Keyboard -> Modifier Keys. Make sure next to Caps Lock, it says Caps Lock and not "No Action".
Have you tried an external USB keyboard? Plug that in, and try the caps lock...
You can also try remapping the caps lock key in that same preference pane - change it to something like the left 'command' key.
Failing that, the next steps might involve shutting down the computer: You'll have to try a reset of the SMC:
- Shut down the Mac.
- Make sure your computer is plugged into power (Plug in the MagSafe or USB-C power adapter to a power source and to your Mac).
- Using the built-in keyboard, press Shift-Control-Option on the left side of the keyboard, then press the power button at the same time.
- Release all keys, then press the power button again to turn on your Mac.
Good luck...