Problem:

Caps Lock key is stuck and you can only type in capital letters on your Mac 

Explanation:

If your key is stuck in the "on" position, you can try this to change the behavior of your Mac’s Caps Lock key—or even disable it altogether, if you wish. 

Temporary Solution - Disable Caps Lock by following the steps below:

• Click the Apple icon in the top-left corner of your desktop, then select System Settings from the drop-down menu. 
• Scroll down and select the Keyboard tab, then click the “Keyboard Shortcuts” under the Keyboard navigation category
• Click the Modifier Keys category. You can then disable the Caps Lock key button for good by choosing "No Action" on the drop-down on the right.
• Click the blue Done button.